U.S. patent number 9,274,462 [Application Number 14/503,567] was granted by the patent office on 2016-03-01 for transfer member and image forming apparatus.
This patent grant is currently assigned to FUJI XEROX CO., LTD.. The grantee listed for this patent is FUJI XEROX CO., LTD.. Invention is credited to Yasuhiro Shimada, Masaaki Yamaura, Tomoaki Yoshioka.
United States Patent |
9,274,462 |
Yoshioka , et al. |
March 1, 2016 |
Transfer member and image forming apparatus
Abstract
A transfer member includes a shaft and a body. When a
measurement member is brought into contact with an outer surface of
the body and voltage applied to the measurement member is changed
by electrically connecting the shaft to ground, a time constant
.tau.v is measured based on a change in electric potential
occurring on a surface of the measurement member. When a first
measurement member is brought into contact with the outer surface,
a second measurement member is brought into contact with the outer
surface while being spaced apart from the first member by a
predetermined distance in a circumferential direction of the outer
surface, and voltage applied to the first member is changed by
electrically connecting the shaft to ground, a time constant .tau.s
is measured based on a change in electric potential occurring on a
surface of the second member. .tau.v is larger than .tau.s.
Inventors: |
Yoshioka; Tomoaki (Kanagawa,
JP), Yamaura; Masaaki (Kanagawa, JP),
Shimada; Yasuhiro (Kanagawa, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
FUJI XEROX CO., LTD. |
Minato-ku, Tokyo |
N/A |
JP |
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Assignee: |
FUJI XEROX CO., LTD. (Tokyo,
JP)
|
Family
ID: |
53754748 |
Appl.
No.: |
14/503,567 |
Filed: |
October 1, 2014 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20150220024 A1 |
Aug 6, 2015 |
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Foreign Application Priority Data
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Feb 5, 2014 [JP] |
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2014-020471 |
Mar 28, 2014 [JP] |
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2014-069020 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G03G
15/1605 (20130101); G03G 15/1685 (20130101); G03G
2215/1614 (20130101); G03G 2215/0129 (20130101) |
Current International
Class: |
G03G
15/16 (20060101) |
Field of
Search: |
;399/101 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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03-100579 |
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Apr 1991 |
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JP |
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09-044002 |
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Feb 1997 |
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JP |
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09-304997 |
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Nov 1997 |
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JP |
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2007-328317 |
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Dec 2007 |
|
JP |
|
Primary Examiner: Laballe; Clayton E
Assistant Examiner: Fenwick; Warren K
Attorney, Agent or Firm: Sughrue Mion, PLLC
Claims
What is claimed is:
1. A transfer member comprising: a shaft; and a body that is
supported by the shaft, wherein when a measurement member extending
in an axial direction of the shaft is brought into contact with an
outer surface of the body and voltage applied to the measurement
member is changed by electrically connecting the shaft to ground, a
time constant measured based on a change in electric potential
occurring on a surface of the measurement member is defined as a
first time constant .tau.v [s], wherein when a first measurement
member extending in the axial direction is brought into contact
with the outer surface of the body, a second measurement member
extending in the axial direction is brought into contact with the
outer surface of the body while being spaced apart from the first
measurement member by a predetermined distance in a circumferential
direction of the outer surface of the body, and voltage applied to
the first measurement member is changed by electrically connecting
the shaft to ground, a time constant measured based on a change in
electric potential occurring on a surface of the second measurement
member is defined as a second time constant .tau.s [s], and wherein
the first time constant .tau.v [s] is larger than the second time
constant .tau.s [s].
2. The transfer member according to claim 1, wherein when an Asker
C hardness of the outer surface of the body is defined as H, the
Asker C hardness H, the first time constant .tau.v [s], and the
second time constant .tau.s [s] are set in the body such that
(1/H).times.0.5<.tau.s<.tau.v is satisfied.
3. The transfer member according to claim 1, wherein the body has
an electrical-conductivity additive blended therein, and wherein
the electrical-conductivity additive is distributed more densely
toward the outer surface of the body from the shaft.
4. The transfer member according to claim 1, wherein the body has
an electrical-conductivity additive blended therein, and wherein
the electrical-conductivity additive is uniformly distributed in
the circumferential direction of the outer surface of the body.
5. The transfer member according to claim 1, wherein the body has
an electrical-conductivity additive blended therein, and wherein
the electrical-conductivity additive is distributed in the body
such that a distance between portions of the
electrical-conductivity additive in the circumferential direction
of the outer surface of the body is shorter than a distance between
portions of the electrical-conductivity additive in a direction
extending from the shaft toward the outer surface of the body.
6. An image forming apparatus comprising: an endless-belt-shaped
image bearing member; the transfer member according to claim 1 that
transfers a visible image on a surface of the image bearing member
onto a medium; and a fixing device that fixes the visible image
transferred on the medium.
7. The image forming apparatus according to claim 6, further
comprising: an electrically-conductive cleaning member that is
disposed downstream, in a rotational direction of the transfer
member, of a facing region in which the transfer member faces the
image bearing member and that cleans the transfer member, the
cleaning member being electrically connected to ground.
8. The image forming apparatus according to claim 7, wherein when a
distance from a downstream end of the facing region in the
rotational direction of the transfer member to a position where the
cleaning member comes into contact with the transfer member is
defined as La [mm], the cleaning member is disposed at a position
that satisfies: La.ltoreq.{(.tau.v/.tau.s)/1.25}.times.12.pi..
9. The image forming apparatus according to claim 7, wherein the
cleaning member includes a first cleaning member that has a brush
portion having a plurality of bristles, and a second plate-shaped
cleaning member that is disposed downstream of the first cleaning
member in the rotational direction of the transfer member and that
cleans the transfer member.
10. The image forming apparatus according to claim 9, wherein the
transfer member has a surface whose ten-point medium height is set
to 2.0 .mu.m or smaller.
11. The image forming apparatus according to claim 9, wherein the
first cleaning member has a rotation shaft and the brush portion
having the bristles extending radially around the rotation shaft,
and wherein the image forming apparatus further comprises: a
supplying section that supplies a lubricant, which lubricates the
transfer member and the second cleaning member, by coming into
contact with the brush portion at an upstream side, in a rotational
direction of the first cleaning member, of a position where the
first cleaning member comes into contact with the transfer
member.
12. The image forming apparatus according to claim 7, further
comprising: a power source that applies voltage between the image
bearing member and the transfer member, the power source being
capable of only applying voltage with a polarity for transferring
the visible image on the surface of the image bearing member onto
the medium.
13. The image forming apparatus according to claim 6, further
comprising: an electricity removal member that removes electricity
from the medium at a downstream side, in a transport direction of
the medium, of a facing region in which the transfer member faces
the image bearing member.
14. The image forming apparatus according to claim 13, wherein the
electricity removal member has an electricity removal section that
removes electricity from the medium, the electricity removal
section being disposed upstream, in a rotational direction of the
transfer member, of an imaginary line that connects a position on
the transfer member, at which a distance Lb [mm] from a central
position of the facing region of the transfer member in the
rotational direction of the transfer member satisfies
Lb={(.tau.v/.tau.s)/1.94}.times.6.pi., and a rotation axis of the
transfer member.
15. An image forming apparatus comprising: an image bearing member;
a latent-image forming device that forms a latent image onto a
surface of the image bearing member; a developing device that
develops the latent image on the surface of the image bearing
member into a visible image; an endless-belt-shaped intermediate
transfer body that is disposed facing the image bearing member; a
first-transfer unit that transfers the visible image on the surface
of the image bearing member onto a surface of the intermediate
transfer body; a support member that supports the intermediate
transfer body in a movable manner; a transfer member that is
disposed facing the intermediate transfer body and that transfers
the visible image on the surface of the intermediate transfer body
onto a medium passing through a facing region in which the transfer
member faces the intermediate transfer body, the transfer member
having a shaft and a body supported by the shaft, wherein when a
measurement member extending in an axial direction of the shaft is
brought into contact with an outer surface of the body and voltage
applied to the measurement member is changed by electrically
connecting the shaft to ground, a time constant measured based on a
change in electric potential occurring on a surface of the
measurement member is defined as a first time constant .tau.v [s],
wherein when a first measurement member extending in the axial
direction is brought into contact with the outer surface of the
body, a second measurement member extending in the axial direction
is brought into contact with the outer surface of the body while
being spaced apart from the first measurement member by a
predetermined distance in a circumferential direction of the outer
surface of the body, and voltage applied to the first measurement
member is changed by electrically connecting the shaft to ground, a
time constant measured based on a change in electric potential
occurring on a surface of the second measurement member is defined
as a second time constant .tau.s [s], wherein a volume resistance
value of the body is defined as Rv [.OMEGA.], wherein a surface
resistance value of the body is defined as Rs [.OMEGA.], wherein a
peripheral speed of the outer surface of the body is defined as v
[mm/s], wherein a length of the facing region in a transport
direction of the medium is defined as L [mm], and wherein the first
time constant .tau.v [s], the second time constant .tau.s [s], the
volume resistance value Rv [.OMEGA.] of the body, the surface
resistance value Rs [.OMEGA.] of the body, the peripheral speed v
[mm/s] of the outer surface of the body, and the length L [mm] of
the facing region in the transport direction of the medium are set
such that (L/v).times.(Rv/Rs)<.tau.s<.tau.v is satisfied; and
a fixing device that fixes the visible image transferred on the
medium.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based on and claims priority under 35 USC 119
from Japanese Patent Application No. 2014-020471 filed Feb. 5, 2014
and Japanese Patent Application No. 2014-069020 filed Mar. 28,
2014.
BACKGROUND
Technical Field
The present invention relates to transfer members and image forming
apparatuses.
SUMMARY
According to an aspect of the invention, there is provided a
transfer member including a shaft and a body that is supported by
the shaft. When a measurement member extending in an axial
direction of the shaft is brought into contact with an outer
surface of the body and voltage applied to the measurement member
is changed by electrically connecting the shaft to ground, a time
constant measured based on a change in electric potential occurring
on a surface of the measurement member is defined as a first time
constant .tau.v [s]. When a first measurement member extending in
the axial direction is brought into contact with the outer surface
of the body, a second measurement member extending in the axial
direction is brought into contact with the outer surface of the
body while being spaced apart from the first measurement member by
a predetermined distance in a circumferential direction of the
outer surface of the body, and voltage applied to the first
measurement member is changed by electrically connecting the shaft
to ground, a time constant measured based on a change in electric
potential occurring on a surface of the second measurement member
is defined as a second time constant .tau.s [s]. The first time
constant .tau.v [s] is larger than the second time constant .tau.s
[s].
BRIEF DESCRIPTION OF THE DRAWINGS
Exemplary embodiments of the present invention will be described in
detail based on the following figures, wherein:
FIG. 1 is an overall view of an image forming apparatus according
to a first exemplary embodiment of the present invention;
FIG. 2 illustrates a relevant part of the image forming apparatus
according to the first exemplary embodiment of the present
invention;
FIG. 3 illustrates a relevant part of a transfer device according
to the first exemplary embodiment of the present invention;
FIGS. 4A and 4B illustrate a transfer member according to the first
exemplary embodiment of the present invention, FIG. 4A illustrating
the length of the transfer member, FIG. 4B being is an enlarged
view illustrating a relevant part of a body thereof;
FIGS. 5A and 5B illustrate a transfer-member manufacturing method
according to the first exemplary embodiment of the present
invention, FIG. 5A illustrating a procedure for manufacturing a
mixture constituting a first layer, FIG. 5B illustrating a
procedure for forming the first layer;
FIGS. 6A to 6C illustrate the transfer-member manufacturing method
according to the first exemplary embodiment of the present
invention, FIG. 6A illustrating a procedure for manufacturing a
resin liquid constituting a second layer, FIG. 6B illustrating a
procedure for forming the second layer, FIG. 6C illustrating a
device used when forming the second layer;
FIGS. 7A and 7B illustrate a second-time-constant measurement
method according to the first exemplary embodiment of the present
invention, FIG. 7A illustrating the configuration of the
measurement method, FIG. 7B illustrating a change in electric
potential relative to time;
FIGS. 8A and 8B illustrate a first-time-constant measurement method
according to the first exemplary embodiment of the present
invention, FIG. 8A illustrating the configuration of the
measurement method, FIG. 8B illustrating a change in electric
potential relative to time;
FIGS. 9A and 9B illustrate a facing region in the image forming
apparatus, FIG. 9A corresponding to FIG. 3, FIG. 9B being a
cross-sectional view taken along line IXB-IXB in FIG. 9A;
FIGS. 10A to 10C illustrate distribution of an
electrical-conductivity additive, FIG. 10A corresponding to FIG.
4B, FIG. 10B being a comparative diagram, FIG. 10C being a
comparative diagram different from FIG. 10B;
FIGS. 11A and 11B illustrate uniform distribution of an
electrical-conductivity additive, FIG. 11A schematically
illustrating a measurement method, FIG. 11B illustrating a
measurement-result determination method;
FIGS. 12A to 12D illustrate a distance in the volume direction and
a distance in the circumferential direction between portions of an
electrical-conductivity additive, FIG. 12A schematically
illustrating a measurement method, FIG. 12B illustrating a
measurement result corresponding to FIG. 10A, FIG. 12C illustrating
a measurement result corresponding to FIG. 10B, FIG. 12D
illustrating a measurement result corresponding to FIG. 10C;
FIG. 13 is an enlarged view illustrating a relevant part of a
transfer member according to a second exemplary embodiment of the
present invention and corresponds to FIG. 4B in the first exemplary
embodiment;
FIGS. 14A and 14B illustrate distribution of an
electrical-conductivity additive in accordance with the second
exemplary embodiment, FIG. 14A corresponding to FIG. 13, FIG. 14B
being a comparative diagram in a case where the
electrical-conductivity additive is uniformly distributed;
FIG. 15 illustrates the relationship between a nip width of a
second-transfer roller and the hardness of the second-transfer
roller;
FIG. 16 illustrates evaluation results of a coefficient A;
FIG. 17 illustrates a maximum coefficient A that satisfies
expression (26) for each speed and each hardness;
FIG. 18 illustrates conditions and experimental results of an
experimental example 1-1, an experimental example 1-2, an
experimental example 1-3, a comparative example 1, and a
comparative example 2;
FIG. 19 illustrates conditions and experimental results of an
experimental example 2-1, an experimental example 2-2, an
experimental example 2-3, an experimental example 2-4, and an
experimental example 2-5;
FIG. 20 illustrates a relevant part of a transfer device according
to a fourth exemplary embodiment of the present invention;
FIGS. 21A and 21B illustrate a comparison between the fourth
exemplary embodiment of the present invention and the related art,
FIG. 21A illustrating the operation of a second-transfer roller
according to the fourth exemplary embodiment, FIG. 21B illustrating
a second-transfer roller according to the related art;
FIG. 22 illustrates the relationship between a potential difference
between the transfer roller and an electrostatic brush and the
remaining amount of developer;
FIG. 23 illustrates a measurement method for measuring a change in
electric potential of the transfer roller;
FIGS. 24A and 24B illustrate measurement results obtained in
accordance with the fourth exemplary embodiment, FIG. 24A
illustrating a time constant in the surface direction and a time
constant in the volume direction, FIG. 24B illustrating the
relationship between the ratio of the time constants and a
reference distance;
FIG. 25 illustrates conditions and experimental results of
experimental examples 3-1 to 3-5 and a comparative example 3-1;
FIGS. 26A and 26B illustrate a relevant part of a transfer device
according to a fifth exemplary embodiment of the present invention,
FIG. 26A corresponding to FIG. 3, FIG. 26B illustrating a detach
saw;
FIG. 27 illustrates an arrangement position of the detach saw
according to the fifth exemplary embodiment of the present
invention;
FIGS. 28A to 28C illustrate a comparison between the fifth
exemplary embodiment of the present invention and the related art,
FIG. 28A illustrating the operation of a second-transfer roller
according to the fifth exemplary embodiment, FIG. 28B illustrating
a second-transfer roller according to the related art, FIG. 28C
illustrating a position where a recording sheet is detached;
FIG. 29 illustrates a measurement method for measuring a change in
electric potential of the transfer roller according to the fifth
exemplary embodiment of the present invention;
FIGS. 30A and 30B illustrate measurement results obtained in
accordance with the fifth exemplary embodiment, FIG. 30A
illustrating a time constant in the surface direction and a time
constant in the volume direction, FIG. 30B illustrating the
relationship between the ratio of the time constants and a
peripheral length; and
FIGS. 31A and 31B illustrate conditions and experimental results of
an experimental example 4-1, an experimental example 4-2, an
experimental example 4-3, a Comparative Example 4-1, and a
comparative example 4-2, FIG. 31A illustrating the conditions, FIG.
31B illustrating the experimental results.
DETAILED DESCRIPTION
Although specific exemplary embodiments of the present invention
will be described below with reference to the drawings, the present
invention is not to be limited to the following exemplary
embodiments.
In order to provide an easier understanding of the following
description, the front-rear direction will be defined as "X-axis
direction" in the drawings, the left-right direction will be
defined as "Y-axis direction", and the up-down direction will be
defined as "Z-axis direction". Moreover, the directions or the
sides indicated by arrows X, -X, Y, -Y, Z, and -Z are defined as
forward, rearward, rightward, leftward, upward, and downward
directions, respectively, or as front, rear, right, left, upper,
and lower sides, respectively.
Furthermore, in each of the drawings, a circle with a dot in the
center indicates an arrow extending from the far side toward the
near side of the plane of the drawing, and a circle with an "x"
therein indicates an arrow extending from the near side toward the
far side of the plane of the drawing.
In the drawings used for explaining the following description,
components other than those for providing an easier understanding
of the description are omitted where appropriate.
First Exemplary Embodiment
Overall Configuration of Printer U According to First Exemplary
Embodiment
FIG. 1 is an overall view of an image forming apparatus according
to a first exemplary embodiment of the present invention.
FIG. 2 illustrates a relevant part of the image forming apparatus
according to the first exemplary embodiment of the present
invention.
Referring to FIGS. 1 and 2, a printer U as an example of the image
forming apparatus according to the first exemplary embodiment
includes a printer body U1, a feeder unit U2 as an example of a
feeding device that feeds a medium to the printer body U1, a
processing unit U3 as an example of a post-processing device that
performs processing on a medium having an image recorded thereon,
an output unit U4 as an example of an output device to which the
medium having the image recorded thereon is output, and an operable
unit UI operable by a user.
Configuration of Marking Unit in First Exemplary Embodiment
Referring to FIGS. 1 and 2, the printer body U1 includes a
controller C that controls the printer U, a communicator (not
shown) that receives image information transmitted from a print
image server COM as an example of an information transmitter
externally connected to the printer U via a dedicated cable (not
shown), and a marking unit U1a as an example of an image recorder
that records an image onto a medium. The print image server COM is
connected, via a line such as a cable or a local area network
(LAN), to a personal computer PC as an example of an image
transmitter that transmits information of an image to be printed in
the printer U.
The marking unit U1a includes photoconductors Py, Pm, Pc, and Pk as
an example of image bearing members for yellow (Y), magenta (M),
cyan (C), and black (K) colors. The photoconductors Py to Pk have
photoconductive dielectric surfaces.
Referring to FIGS. 1 and 2, in the rotational direction of the
photoconductor Pk for the black color, a charger CCk, an exposure
unit ROSk as an example of a latent-image forming unit, a
developing unit Gk, a first-transfer roller Tlk as an example of a
first-transfer unit, and a photoconductor cleaner CLk as an example
of an image-bearing-member cleaner are arranged around the
photoconductor Pk.
Likewise, chargers CCy, CCm, and CCc, exposure units ROSy, ROSm,
and ROSc, developing units Gy, Gm, and Gc, first-transfer rollers
T1y, T1m, and T1c, and photoconductor cleaners CLy, CLm, and CLc
are respectively arranged around the remaining photoconductors Py,
Pm, and Pc.
Toner cartridges Ky, Km, Kc, and Kk as an example of containers
that accommodate therein developers to be supplied to the
developing units Gy to Gk are detachably supported above the
marking unit U1a.
An intermediate transfer belt B as an example of an intermediate
transfer body and an image bearing member is disposed below the
photoconductors Py to Pk. The intermediate transfer belt B is
interposed between the photoconductors Py to Pk and the
first-transfer rollers T1y to T1k. The undersurface of the
intermediate transfer belt B is supported by a drive roller Rd as
an example of a drive member, a tension roller Rt as an example of
a tension applying member, a working roller Rw as an example of a
meander prevention member, multiple idler rollers Rf as an example
of driven members, a backup roller T2a as an example of a
second-transfer opposing member, multiple retracting rollers R1 as
an example of movable members, and the aforementioned
first-transfer rollers T1y to T1k.
A belt cleaner CLB as an example of an intermediate-transfer-body
cleaner is disposed on the top surface of the intermediate transfer
belt B near the drive roller Rd.
A second-transfer roller T2b as an example of a second-transfer
member is disposed facing the backup roller T2a with the
intermediate transfer belt B interposed therebetween. The backup
roller T2a is in contact with a contact roller T2c as an example of
a contact member for applying voltage having a reversed polarity
relative to the charge polarity of the developers to the backup
roller T2a.
The backup roller T2a, the second-transfer roller T2b, and the
contact roller T2c constitute a second-transfer unit T2 according
to the first exemplary embodiment. The first-transfer rollers T1y
to T1k, the intermediate transfer belt B, the second-transfer unit
T2, and the like constitute a transfer device T1+B+T2 according to
the first exemplary embodiment.
Feed trays TR1 to TR3 as an example of containers that accommodate
therein recording sheets S as an example of media are provided
below the second-transfer unit T2. A pickup roller Rp as an example
of a fetching member and a separating roller Rs as an example of a
separating member are disposed at the upper left side of each of
the feed trays TR1 to TR3. A transport path SH that transports each
recording sheet S extends from the separating roller Rs. Multiple
transport rollers Ra as an example of transport members that
transport each recording sheet S downstream are arranged along the
transport path SH.
A registration roller Rr as an example of an adjusting member that
adjusts the timing for transporting each recording sheet S toward
the second-transfer unit T2 is disposed at the downstream side of
the transport rollers Ra.
The feeder unit U2 is similarly provided with components, such as
feed trays TR4 and TR5 that have configurations similar to those of
the feed trays TR1 to TR3, the pickup rollers Rp, the separating
rollers Rs, and the transport rollers Ra. A transport path SH from
the feed trays TR4 and TR5 merges with the transport path SH in the
printer body U1 at the upstream side of the registration roller
Rr.
Multiple transport belts HB as an example of a medium transport
device are arranged at the downstream side of the second-transfer
roller T2b in the transport direction of the recording sheet S.
A fixing device F is disposed at the downstream side of the
transport belts HB in the transport direction of the recording
sheet S. The fixing device F includes a heating roller Fh as an
example of a heating member and a pressing roller Fp as an example
of a pressing member. The heating roller Fh accommodates therein a
heater as an example of a heat source.
A cooling device Co is disposed within the processing unit U3 at
the downstream side of the fixing device F.
An image reading device Sc that reads an image recorded on the
recording sheet S is disposed at the downstream side of the cooling
device Co.
A transport path SH extending toward the output unit U4 is formed
at the downstream side of the image reading device Sc. An inversion
path SH2 as an example of a transport path is formed inside the
processing unit U3. The inversion path SH2 diverges downward from
the transport path SH. A first gate GT1 as an example of a
transport-direction switching member is disposed at the diverging
point between the transport path SH and the inversion path SH2.
Multiple switchback rollers Rb as an example of transport members
that are rotatable in forward and reverse directions are arranged
along the inversion path SH2. A connection path SH3 as an example
of a transport path that diverges from an upstream section of the
inversion path SH2 and merges with the transport path SH at the
downstream side of the diverging point of the inversion path SH2 is
formed at the upstream side of the switchback rollers Rb. A second
gate GT2 as an example of a transport-direction switching member is
disposed at the diverging point between the inversion path SH2 and
the connection path SH3.
A circulation path SH4 as an example of a transport path is
disposed below the inversion path SH2. The circulation path SH4
diverges from the inversion path SH2, extends leftward, and merges
with the transport path SH in the printer body U1 at the upstream
side of the registration roller Rr. Transport rollers Ra as an
example of transport members are arranged along the circulation
path SH4. A third gate GT3 as an example of a transport-direction
switching member is disposed at the diverging point of the
circulation path SH4 from the inversion path SH2.
In the output unit U4, a stacker tray TRh as an example of a
container on which output recording sheets S are stacked is
disposed, and an output path SH5 diverging from the transport path
SH extends toward the stacker tray TRh. The transport path SH in
the first exemplary embodiment is configured such that, when an
additional output unit (not shown) or an additional post-processing
device (not shown) is attached to the right side of the output unit
U4, the transport path SH is capable of transporting the recording
sheet S to the added unit or device.
Operation of Marking Unit
When the printer U receives image information transmitted from the
personal computer PC via the print image server COM, the printer U
commences a job, which is image forming operation. When the job
commences, the photoconductors Py to Pk, the intermediate transfer
belt B, and the like rotate.
The photoconductors Py to Pk are rotationally driven by a drive
source (not shown).
The chargers CCy to CCk receive a predetermined voltage so as to
charge the surfaces of the photoconductors Py to Pk.
The exposure units ROSy to ROSk output laser beams Ly, Lm, Lc, and
Lk as an example of latent-image write-in light in accordance with
a control signal from the controller C so as to write electrostatic
latent images onto the charged surfaces of the photoconductors Py
to Pk.
The developing units Gy to Gk develop the electrostatic latent
images on the surfaces of the photoconductors Py to Pk into visible
images.
The toner cartridges Ky to Kk supply developers as the developers
are consumed in the developing process performed in the developing
units Gy to Gk.
The first-transfer rollers T1y to T1k receive a first-transfer
voltage with a reversed polarity relative to the charge polarity of
the developers so as to transfer the visible images on the surfaces
of the photoconductors Py to Pk onto the surface of the
intermediate transfer belt B.
The photoconductor cleaners CLy to CLk clean the surfaces of the
photoconductors Py to Pk after the first-transfer process by
removing residual developers therefrom.
When the intermediate transfer belt B passes through first-transfer
regions facing the photoconductors Py to Pk, Y, M, C, and K images
are transferred and superposed on the intermediate transfer belt B
in that order, and the intermediate transfer belt B subsequently
travels through a second-transfer region Q4 facing the
second-transfer unit T2. When a monochrome image is to be formed,
an image of a single color is transferred onto the intermediate
transfer belt B and is transported to the second-transfer region
Q4.
In accordance with the size of the received image information, the
designated type of recording sheets S, the sizes and types of
accommodated recording sheets S, and so on, one of the pickup
rollers Rp feeds recording sheets S from the corresponding one of
the feed trays TR1 to TR5 from which the recording sheets S are to
be fed.
The corresponding separating roller Rs separates the recording
sheets S fed by the pickup roller Rp in a one-by-one fashion.
The registration roller Rr feeds the recording sheet S in
accordance with a timing at which the image on the surface of the
intermediate transfer belt B is transported to the second-transfer
region Q4.
In the second-transfer unit T2, a predetermined second-transfer
voltage having the same polarity as the charge polarity of the
developers is applied to the backup roller T2a via the contact
roller T2c so that the image on the intermediate transfer belt B is
transferred onto the recording sheet S.
The belt cleaner CLB cleans the surface of the intermediate
transfer belt B after the image transfer process performed at the
second-transfer region Q4 by removing residual developers
therefrom.
The recording sheet S having the image transferred thereon at the
second-transfer unit T2 is transported downstream by the transport
belts HB while being supported on the surfaces thereof.
The fixing device F heats and presses the recording sheet S passing
through a fixing region where the heating roller Fh and the
pressing roller Fp are in contact with each other so as to fix an
unfixed image onto the surface of the recording sheet S.
The cooling device Co cools the recording sheet S heated by the
fixing device F.
The image reading device Sc reads the image from the surface of the
recording sheet S having passed through the cooling device Co. The
read image may be compared with a document image so as to be used
for, for example, detecting print errors or detecting
misregistration of the image.
In the case of duplex printing, the recording sheet S having passed
through the image reading device Sc is transported to the inversion
path SH2 by activation of the first gate GT1 and is switched back
so as to be transported again to the registration roller Rr via the
circulation path SH4, whereby printing is performed on the second
face of the recording sheet S.
The recording sheet S to be output to the output unit U4 is
transported along the transport path SH so as to be output onto the
stacker tray TRh. In this case, if the recording sheet S to be
output to the stacker tray TRh is in an inverted state, the
recording sheet S is temporarily transported to the inversion path
SH2 from the transport path SH. After the trailing edge of the
recording sheet S in the transport direction thereof passes through
the second gate GT2, the second gate GT2 is switched and the
switchback rollers Rb are rotated in the reverse direction so that
the recording sheet S is transported along the connection path SH3
toward the stacker tray TRh.
When multiple recording sheets S are stacked on the stacker tray
TRh, a stacker plate TRh1 automatically moves upward or downward in
accordance with the number of stacked recording sheets S so that
the uppermost sheet is disposed at a predetermined height.
Intermediate Transfer Body and Second-Transfer Unit According to
First Exemplary Embodiment
FIG. 3 illustrates a relevant part of the transfer device according
to the first exemplary embodiment of the present invention.
Referring to FIGS. 1 to 3, the backup roller T2a as an example of a
support member and an opposed member is disposed in the
second-transfer region Q4. The backup roller T2a has a metallic
shaft 1 as an example of a rotation shaft. The shaft 1 extends in
the front-rear direction. The shaft 1 supports a roller layer 2 as
an example of an opposed-member body. The roller layer 2 has a base
layer 3 and a surface layer 4 supported by the outer side of the
base layer 3. The base layer 3 is composed of rubber as an example
of an elastic material. The rubber of the base layer 3 has an
electrical-conductivity additive blended therein. The surface layer
4 is composed of resin. The surface layer 4 has an
electrical-conductivity additive blended therein. The roller layer
2 is set to a predetermined hardness H1.
The backup roller T2a supports the intermediate transfer belt B,
which is an endless belt, as an example of an intermediate transfer
body according to the first exemplary embodiment. The intermediate
transfer belt B is composed of resin having an
electrical-conductivity additive blended therein.
FIGS. 4A and 4B illustrate a transfer member according to the first
exemplary embodiment of the present invention. Specifically, FIG.
4A illustrates the length of the transfer member, and FIG. 4B is an
enlarged view illustrating a relevant part of a body thereof.
Referring to FIGS. 3 to 4B, the second-transfer roller T2b as an
example of the transfer member according to the first exemplary
embodiment is disposed at a position facing to the backup roller
T2a with the intermediate transfer belt B interposed therebetween.
The second-transfer roller T2b has a metallic shaft 6 as an example
of a shaft. The shaft 6 extends in the front-rear direction. The
shaft 6 supports a roller layer 7 as an example of a body. The
roller layer 7 has a length .lamda.2 that is shorter, in the
front-rear direction, than a length .lamda.1 of the shaft 6. The
roller layer 7 has a base layer 8 as an example of a first layer.
The roller layer 7 also has a surface layer 9, as an example of a
second layer, which is supported radially outward than the base
layer 8. Thus, the layers 8 and 9 have radially-inward inner
surfaces 8b and 9b and radially-outward outer surfaces 8a and 9a,
respectively.
In FIG. 4B, the base layer 8 is composed of rubber 11. The rubber
11 has an electrical-conductivity additive 12 blended therein. The
surface layer 9 is composed of resin 13. The resin 13 of the
surface layer 9 has an electrical-conductivity additive 14 blended
therein. In the first exemplary embodiment, the percentage of
electrical-conductivity additive blended in the surface layer 9 is
higher than that in the base layer 8. In the layers 8 and 9
according to the first exemplary embodiment, the
electrical-conductivity additives 12 and 14 are distributed within
the layers 8 and 9, respectively, with low unevenness. Therefore,
in contrast to a transfer roller in the related art in which the
electrical-conductivity additive normally decreases toward the
outer layer, the electrical-conductivity additive in the surface
layer 9 is blended therein with higher density than in the base
layer 8 in the first exemplary embodiment.
In FIGS. 3 to 4B, the roller layer 7 is given a hardness H2 in
accordance with the hardness H1 of the backup roller T2a. In the
first exemplary embodiment, the difference between the hardness H1
and the hardness H2 is small. Thus, in the second-transfer region
Q4 in the first exemplary embodiment, a region formed between the
backup roller T2a and the second-transfer roller T2b, that is, a
nip region 16, is formed into a flat plane. In other words, the nip
region 16 where the intermediate transfer belt B and the
second-transfer roller T2b face each other is formed into a flat
plane. The second-transfer roller T2b receives load such that the
length of the nip region 16 in the transport direction of a
recording sheet S, that is, a nip width, is equal to a
predetermined length L.
Transfer-Member Manufacturing Method
Shaft 6
The shaft 6 is an electrically-conductive member functioning as a
support member and an electrode of the second-transfer roller
T2b.
The shaft 6 is composed of a metallic material such as iron (such
as free-cutting steel), copper, brass, stainless steel, aluminum,
or nickel.
Other examples of the shaft 6 include a member (such as a resin or
ceramic member) whose outer surface is coated with metal and a
member (such as a resin or ceramic member) having an
electrically-conductive agent distributed therein.
The shaft 6 may be a hollow member (tubular member) or a non-hollow
member.
Base Layer 8
The base layer 8 is an electrically-conductive layer and includes a
rubber material (elastic material) 21 and an
electrical-conductivity additive 22. The base layer 8 may contain
other additives. Furthermore, the base layer 8 may be an
electrically-conductive foamed elastic layer or an
electrically-conductive non-foamed elastic layer. However, in view
of prevention of liquid entering a foam material when forming the
surface layer 9, a non-foamed elastic layer is desired.
The rubber material (elastic material) 21 is, for example, an
elastic material at least having a double bond within its chemical
structure.
Specific examples of the rubber material 21 include isoprene
rubber, chloroprene rubber, epichlorohydrin rubber, butyl rubber,
polyurethane, silicone rubber, fluorocarbon rubber,
styrene-butadiene rubber, butadiene rubber, nitrile rubber,
ethylene-propylene rubber, epichlorohydrin-ethylene oxide copolymer
rubber, epichlorohydrin-ethylene oxide-allyl glycidyl ether
copolymer rubber, ethylene-propylene-diene terpolymer (EPDM),
acrylonitrile-butadiene copolymer rubber (NBR), natural rubber, and
rubber containing a mixture of these materials.
Of the above examples of the rubber material 21, suitable examples
include polyurethane, EPDM, epichlorohydrin-ethylene oxide
copolymer rubber, epichlorohydrin-ethylene oxide-allyl glycidyl
ether copolymer rubber, NBR, and rubber containing a mixture of
these materials.
The electrical-conductivity additive 22 is to be used, for example,
when the rubber material 21 has low electrical conductivity or when
the rubber material 21 does not have electrical conductivity.
Examples of the electrical-conductivity additive 22 include an
electronic conductive agent and an ionic conductive agent.
For example, the electronic conductive agent may be a powder
material, examples of which include carbon black, such as Ketjen
black or acetylene black; pyrolytic carbon and graphite;
electrically-conductive metal of various kinds, such as aluminum,
copper, nickel, or stainless steel, or an alloy thereof;
electrically-conductive metal oxide of various kinds, such as tin
oxide, indium oxide, titanium oxide, a tin oxide-antimony oxide
solid solution, or a tin oxide-indium oxide solid solution; and an
insulating material whose surface has been processed to have
electrical conductivity.
Specific examples of carbon black include "Special Black 350",
"Special Black 100", "Special Black 250", "Special Black 5",
"Special Black 4", "Special Black 4A", "Special Black 550",
"Special Black 6", "Color Black FW200", "Color Black FW2", "Color
Black FW2V", which are manufactured by Degussa Corporation, and
"MONARCH 1000", "MONARCH 1300", "MONARCH 1400", "MOGUL-L", and
"REGAL 400R", which are manufactured by Cabot Corporation.
The electronic conductive agent may be used alone or may be used by
combining two or more kinds thereof.
For example, the content of the electronic conductive agent often
ranges between 1 part by mass and 30 parts by mass relative to 100
parts by mass of the rubber material.
Examples of the ionic conductive agent include quaternary ammonium
salt (e.g., perchlorate, such as lauryl trimethyl ammonium, stearyl
trimethyl ammonium, octa dodecyl trimethyl ammonium, dodecyl
trimethyl ammonium, hexadecyl trimethyl ammonium, and modified
fatty acid-dimethyl ethyl ammonium, chlorate salt, fluoboric acid
salt, sulfate salt, ethyl sulfate salt, halogenated benzyl salt
(such as benzyl bromide salt or benzyl chloride salt), aliphatic
sulfonate salt, fatty alcohol sulfate salt, fatty-alcohol
ethylene-oxide-added sulfate salt, fatty alcohol phosphate salt,
fatty-alcohol ethylene-oxide-added phosphate salt, various kinds of
betaine, fatty alcohol ethylene oxide, polyethylene glycol fatty
acid ester, and polyalcohol fatty acid ester.
The ionic conductive agent may be used alone or may be used by
combining two or more kinds thereof.
For example, the content of the ionic conductive agent often ranges
between 0.1 parts by mass and 5.0 by mass relative to 100 parts by
mass of the rubber material.
Other additives that may be added to the rubber layer generally
include, for example, a foaming agent, a foaming assistant, a
softening agent, a plasticizing agent, a curing agent, a
vulcanizing agent 23, a vulcanization accelerator 24, an
antioxidant, a surfactant, a coupling agent, and a filler (such as
silica or calcium carbonate).
Surface Layer 9
The surface layer 9 contains a resin material 31 and an
electrical-conductivity additive 32. The surface layer 9 may also
contain other additives.
Examples of the resin material 31 include acrylic resin, cellulose
resin, polyamide resin, copolymer nylon, polyurethane resin,
polycarbonate resin, polyester resin, polyethylene resin, polyvinyl
resin, polyarylate resin, styrene-butadiene resin, melamine resin,
epoxy resin, urethane resin, silicone resin, fluoro-resin (such as
a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer,
tetrafluoroethylene hexafluoropropylene copolymer, or
polyvinylidene fluoride), and urea resin.
Copolymer nylon includes one of or multiple kinds of nylon 610,
nylon 11, and nylon 12 as a polymer unit. Other examples of polymer
unit included in this copolymer include nylon 6 and nylon 66. The
resin material 31 may be curable resin 33 cured by using a curing
agent 34.
Examples of the electrical-conductivity additive 32 include an
electronic conductive agent and an ionic conductive agent. Examples
of the electrical-conductivity additive 32 are similar to those of
the electrical-conductivity additive 22 in the description of the
base layer 8.
Other additives that may be added to the resin layer generally
include a plasticizing agent, a curing agent, a softening agent, an
antioxidant, and a surfactant.
In view of suppressing both cracking and scratches by adjusting
Young's modulus and micro-hardness of the roller surface, the
surface layer 9 may be a resin layer composed of constituents
including the curable resin 33, the curing agent 34, and carbon
black. In particular, the surface layer 9 may be a resin layer
formed of a cured film composed of constituents including resin
(curable resin) having a functional group reactable with an
isocyanate group, an isocyanate curing agent, and carbon black.
The resin layer formed of this cured film is suitable due to the
following reasons. Lower Young's modulus of the roller surface is
achieved in accordance with, for example, the type, the amount, and
the calcination temperature (curing temperature) of the curing
agent, so that the occurrence of cracking is reduced. In addition,
the micro-hardness of the roller surface is increased in accordance
with the amount of carbon black, so that the occurrence of
scratches is reduced.
Suitable examples of the curable resin 33 include a
tetrafluoroethylene-vinyl monomer copolymer, polyamide,
polyurethane, polyvinylidene fluoride, a tetrafluoroethylene
copolymer, polyester, polyimide, silicone resin, acrylic resin,
polyvinyl butyral, an ethylene tetrafluoroethylene copolymer,
melamine resin, fluoro-rubber, epoxy resin, polycarbonate,
polyvinyl alcohol, cellulose, polyvinylidene chloride, polyvinyl
chloride, polyethylene, and an ethylene-vinyl acetate
copolymer.
In particular, examples of resin having a functional group
reactable with an isocyanate group include acrylic polyol,
polyester polyol, polyether polyol, polycarbonate polyol,
polycaprolactone polyol, and polyolefin polyol, each of which has a
hydroxyl group within a molecule. For the purpose of functional
improvements, for example, a fluoroolefin copolymer (such as a
tetrafluoroethylene-vinyl monomer copolymer) or a vinyl fluoride
copolymer may be used.
A low molecular-weight polyisocyanate compound having an isocyanate
group at a molecular end thereof may be used as the curing agent
34. Specific examples include Coronate L, Coronate 2030, Coronate
HX, Coronate HL (manufactured by Nippon Polyurethane Industry Co.,
Ltd.), Desmodur L, Desmodur N 3300, Desmodur HT (manufactured by
Bayer Holding Ltd.), Takenate D-102, Takenate D-160N, Takenate
D-170N (manufactured by Takeda Pharmaceutical Company Limited),
Sumidur N3300 (manufactured by Sumika Bayer Urethane Co., Ltd.),
T1890 (manufactured by Degussa Corporation), and diphenylmethane
diisocyanate (MDI).
The isocyanate group (NCO group) and the hydroxyl group (OH group)
within the polyol may be mixed such that the molar ratio (NCO/OH,
R-value) of the isocyanate group (NCO group) to the hydroxyl group
(OH group) ranges between 0.2 and 1.5, desirably between 0.3 and
1.3, and more desirably between 0.9 and 1.1. Furthermore, in
addition to a reaction inhibitor and a metallic catalyst, additives
for controlling physical properties, such as a surfactant, a foam
stabilizer, a defoaming agent, a fire retardant, a plasticizing
agent, a colorant, dye, a stabilizer, an antibacterial agent, and a
filler, may be included.
The surface layer 9 is formed by preparing an application liquid
while distributing each component in a solvent 36, applying the
application liquid over the base layer 8, and then drying and
baking (curing), where appropriate, the application liquid.
For the preparation of the application liquid, a colliding-type
distribution device, such as a jet mill or a homogenizer, may be
used for enhancing the distribution of the electrical-conductivity
additive (carbon black). By enhancing the distribution of the
electrical-conductivity additive (carbon black), the content of the
electrical-conductivity additive within the surface layer 9 and the
micro-hardness thereof may be increased while suppressing an
excessive increase in resistivity of the surface layer 9.
As the solvent 36, a normal organic solvent may be used alone or a
mixture of two or more kinds of organic solvents may be used.
Examples of organic solvents include butyl acetate, methanol,
ethanol, n-propanol, n-butanol, benzyl alcohol, methyl cellosolve,
ethyl cellosolve, acetone, methyl ethyl ketone, cyclohexanone,
n-butyl acetate, dioxane, tetrahydrofuran, chlorobenzene, and
toluene.
Formation of Base Layer 8
FIGS. 5A and 5B illustrate a transfer-member manufacturing method
according to the first exemplary embodiment of the present
invention. Specifically, FIG. 5A illustrates a procedure for
manufacturing a mixture constituting the first layer, and FIG. 5B
illustrates a procedure for forming the first layer.
In FIG. 5A, a mixture 29 as an example of a material constituting
the base layer 8 according to the first exemplary embodiment is
manufactured in accordance with the following process. First, the
rubber material 21 and the electrical-conductivity additive 22 are
mixed together so that a mixture 27 is obtained. Then, the
vulcanizing agent 23 and the vulcanization accelerator 24 are added
to the mixture 27 so that a mixture 28 is obtained. Subsequently,
the mixture 28 is kneaded by using an open roller as an example of
a kneading device, so that the mixture 29 is obtained.
Referring to FIG. 5B, the mixture 29 is then wrapped around the
shaft 6. Subsequently, the shaft 6 is increased in temperature. The
mixture 29 wrapped around the shaft 6 is then vulcanized and foamed
for a predetermined time period. Consequently, the base layer 8,
which has elasticity, is formed around the shaft 6. Then, the outer
surface 8a of the base layer 8 is ground so that the base layer 8
is machined to a predetermined outside diameter, whereby a roller
equipped with the base layer 8 is obtained.
Formation of Surface Layer 9
FIGS. 6A to 6C illustrate the transfer-member manufacturing method
according to the first exemplary embodiment of the present
invention. Specifically, FIG. 6A illustrates a procedure for
manufacturing a resin liquid constituting the second layer, FIG. 6B
illustrates a procedure for forming the second layer, and FIG. 6C
illustrates a device used when forming the second layer.
In FIG. 6A, a resin liquid 43 as an example of a resin liquid
constituting the second layer is manufactured in accordance with
the following process. First, the curable resin 33 and the
electrical-conductivity additive 32 are injected into the solvent
36 so that a resin liquid 37 is produced. The resin liquid 37
undergoes a distribution process in a jet-mill distribution device
38 as an example of a distribution device. The resin liquid 37
having undergone the distribution process is made to pass through
stainless-steel mesh 39 as an example of a removing member. Thus,
foreign matter in the resin liquid 37, aggregates in the
electrical-conductivity additive 32, and the like are removed
therefrom. The resin liquid 37 from which foreign matter has been
removed undergoes a vacuum degassing process. Thus, air is removed
from the resin liquid 37. Consequently, a degassed resin liquid 41
is manufactured. The degassed resin liquid 41 is mixed with the
curing agent 34 so that a resin liquid 42 is manufactured. Then,
the electrical-conductivity additive 32 is blended into the resin
liquid 42. As a result, the resin liquid 43 for the surface layer
according to the first exemplary embodiment is manufactured.
Referring to FIG. 6B, the outer surface 8a of the base layer 8
around the shaft 6 is coated with the surface-layer resin liquid
43. In the first exemplary embodiment, spray coating is performed
as an example of a coating method. Specifically, the shaft 6 is
supported in a state where the axial direction thereof is aligned
with the horizontal direction. Then, the shaft 6 is rotated at a
predetermined rotation speed u1. Thus, the base layer 8 rotates
together with the shaft 6. Then, the surface-layer resin liquid 43
is sprayed onto the outer surface 8a of the rotating base layer 8
from a spray nozzle 51 as an example of a feeder. In this case, the
nozzle 51 is moved at a predetermined relative speed in the axial
direction of the shaft 6. Thus, the outer surface 8a of the base
layer 8 becomes coated with the sprayed resin liquid 43, whereby a
layer is formed. When the layer of the resin liquid 43 is formed,
the layer is baked by being heated for a predetermined time period.
In the first exemplary embodiment, the shaft 6 rotates even during
the heating process. Consequently, the surface layer 9 of the
roller layer 7 is formed, whereby the second-transfer roller T2b is
formed.
Transfer-Member Measurement Method
FIGS. 7A and 7B illustrate a second-time-constant measurement
method according to the first exemplary embodiment of the present
invention. Specifically, FIG. 7A illustrates the configuration of
the measurement method, and FIG. 7B illustrates a change in
electric potential relative to time.
In the second-transfer roller T2b, a time constant .tau.s in the
surface direction is set as an example of a second time constant.
The time constant .tau.s in the surface direction is measured with
the following configuration.
In FIG. 7A, a first electrically-conductive metallic plate 61 and a
second electrically-conductive metallic plate 62 as examples of
measurement members are disposed on the outer surface of the
second-transfer roller T2b, that is, the outer surface 9a of the
roller layer 7. The first metallic plate 61 and the second metallic
plate 62 have identical plate-like shapes. Each of the metallic
plates 61 and 62 has a length .lamda.3 in the front-rear direction
that is longer than a length .lamda.2 of the roller layer 7 of the
second-transfer roller T2b. Furthermore, each of the metallic
plates 61 and 62 has a length .lamda.4 in the left-right direction,
that is, the thickness direction thereof. The metallic plates 61
and 62 are supported such that surfaces 61a and 62a thereof having
sides with the lengths .lamda.3 and .lamda.4 are pressed against
the outer surface 9a of the roller layer 7.
The second metallic plate 62 is spaced apart from the first
metallic plate 61 in the circumferential direction of the outer
surface 9a of the roller layer 7. Specifically, in FIG. 7A, the
first metallic plate 61 and the second metallic plate 62 are
disposed such that the peripheral length between a right surface
61b of the first metallic plate 61 and a left surface 62b of the
second metallic plate 62 is set to a predetermined length .lamda.5.
An insulating member 63 is disposed between the first metallic
plate 61 and the second metallic plate 62. Thus, the right surface
61b of the first metallic plate 61 and the left surface 62b of the
second metallic plate 62 are insulated from each other.
The shaft 6 of the second-transfer roller T2b is electrically
connected to ground. On the other hand, the first metallic plate 61
is connected to a direct-current voltage source 64 as an example of
a power source. The direct-current voltage source 64 applies
voltage to the first metallic plate 61. The direct-current voltage
source 64 is switchable between an on state in which the
direct-current voltage source 64 applies a predetermined voltage V0
and an off state in which the direct-current voltage source 64
stops applying the voltage. Referring to FIG. 7A, in the first
exemplary embodiment, a surface electrometer 66 is disposed in
correspondence with a right surface 62c of the second metallic
plate 62. The surface electrometer 66 measures an electric
potential V of the right surface 62c of the second metallic plate
62.
When the direct-current voltage source 64 switches from the off
state to the on state, the roller layer 7 of the second-transfer
roller T2b receives voltage via the first metallic plate 61. Thus,
an electrical change occurs in the surface direction and the volume
direction of the roller layer 7 as the voltage application starts.
Specifically, when an electrical change occurs in the surface
direction of the roller layer 7, the electric potential V of the
second metallic plate 62 changes. In this case, as shown in FIG.
7B, the surface electrometer 66 measures the electric potential V,
which changes from zero toward a certain electric potential V1. In
FIG. 7B, the abscissa axis denotes time t elapsed since the start
of application of the voltage V0, and the ordinate axis denotes the
measured electric potential V.
This phenomenon in which the electric potential V changes from zero
to V1 is known as a so-called transient phenomenon. The electric
potential V is known to change based on expression (1) shown below
when Napier's constant is defined as e, the time elapsed since the
start of voltage application is defined as t, and the time constant
in the surface direction of the second-transfer roller T2b is
defined as .tau.s: V=V1.times.(1-e.sup.(-t/.tau.s)) (1)
Therefore, it is clear from expression (1) that, when the time t is
sufficiently large, the value of e.sup.(-t/.tau.s) is small and the
electric potential V hardly changes. Thus, when the time t is
sufficiently large, the electric potential V is stable such that
V.apprxeq.V1.
Furthermore, by substituting the time t for the time constant
.tau.s in the surface direction in expression (1), expression (2)
shown below is obtained:
.times..times..times..times.e.tau..tau..times..times..times..times.e.time-
s..times..times..times.e.times..times..times..times.ee.apprxeq..times..tim-
es..times..times..apprxeq..times..times..times..times.
##EQU00001##
Therefore, it is clear from expression (2) that, when time .tau.s
elapses from the start of application of the voltage V0, the
electric potential V becomes a value of about 63% of the electric
potential V1.
Referring to FIG. 7B, in the first exemplary embodiment, a change
in the electric potential V of the second metallic plate 62 since
the start of application of the voltage V0 is measured.
Furthermore, an electric potential V measured at a predetermined
sufficiently large time T1 is defined as an electric potential V1.
Moreover, the time t when the electric potential V of the second
metallic plate 62 becomes 63% of the electric potential V1 is
determined. Then, the determined time t is set as the time constant
.tau.s in the surface direction.
FIGS. 8A and 8B illustrate a first-time-constant measurement method
according to the first exemplary embodiment of the present
invention. Specifically, FIG. 8A illustrates the configuration of
the measurement method, and FIG. 8B illustrates a change in
electric potential relative to time.
In the second-transfer roller T2b, a time constant .tau.v in the
volume direction is set as an example of a first time constant. The
time constant .tau.v in the volume direction is measured with the
following configuration.
In FIG. 8A, the same components 61, 64, and 66 used for measuring
the time constant .tau.s in the surface direction are used except
that the second metallic plate 62 and the insulating member 63 are
omitted. Specifically, when measuring the time constant .tau.v in
the volume direction, an electric potential V of the first metallic
plate 61 supported by being pressed against the outer surface 9a of
the second-transfer roller T2b is measured in place of the electric
potential V of the second metallic plate 62. Referring to FIG. 8A,
in the first exemplary embodiment, the surface electrometer 66 is
disposed in correspondence with the right surface 61b of the first
metallic plate 61. The surface electrometer 66 measures the
electric potential V of the right surface 61b of the first metallic
plate 61.
When the direct-current voltage source 64 switches from the on
state to the off state, the direct-current voltage source 64 stops
applying voltage to the roller layer 7 of the second-transfer
roller T2b. Thus, an electrical change occurs in the surface
direction and the volume direction of the roller layer 7 as the
voltage application stops. As an electrical change occurs in the
volume direction of the roller layer 7, the electric potential V of
the first metallic plate 61 changes. Thus, as shown in FIG. 8B, the
surface electrometer 66 measures the electric potential V, which
changes from an initial electric potential V2 toward zero. In FIG.
8B, the abscissa axis denotes time t elapsed since the stoppage of
voltage application, and the ordinate axis denotes the measured
electric potential V.
This phenomenon in which the electric potential V changes from V2
to zero is known as a so-called transient phenomenon. The electric
potential V is known to change based on expression (3) shown below
when the time constant in the volume direction of the
second-transfer roller T2b is defined as .tau.v:
V=V2.times.e.sup.(-t/.tau.v) (3)
By substituting the time t for the time constant .tau.v in the
volume direction in expression (3), expression (4) shown below is
obtained:
.times..times..times..times..tau..tau..times..times..times..times.e.times-
..times..apprxeq..times..times..times..times..apprxeq..times..times..times-
..times. ##EQU00002##
Therefore, it is clear from expression (4) that, when time .tau.v
elapses from the stoppage of voltage application, the electric
potential V becomes a value of about 37% of the initial electric
potential V2.
Referring to FIG. 8B, in the first exemplary embodiment, a change
in the electric potential V of the first metallic plate 61 since
the stoppage of voltage application is measured. Furthermore, an
electric potential V when the time t corresponding to the on state
is equal to zero is defined as an initial electric potential V2.
Moreover, the time t when the electric potential V of the first
metallic plate 61 becomes 37% of the electric potential V2 is
determined. Then, the determined time t is set as the time constant
.tau.v in the volume direction.
Value Settings of Transfer Member
In the second-transfer roller T2b, the time constant .tau.s [s] in
the surface direction and the time constant .tau.v [s] in the
volume direction are set so as to satisfy the relationship
expressed by expression (11) shown below: .tau.s<.tau.v (11)
In particular, referring to FIG. 3, in the first exemplary
embodiment, when the length of the nip region 16 in the sheet
transport direction in the second-transfer region Q4 is denoted by
L [mm] and the rotation speed as an example of a peripheral speed
of the outer surface of the second-transfer roller T2b is denoted
by v [mm/s], the second-transfer roller T2b is set such that the
time constant .tau.s [s] in the surface direction, the time
constant .tau.v [s] in the volume direction, a volume resistance
value Rv [.OMEGA.] of the roller layer 7, and a surface resistance
value Rs [.OMEGA.] of the roller layer 7 satisfy the relationship
expressed by expression (12) shown below:
(L/v).times.(Rv/Rs)<.tau.s<.tau.v (12)
Operation of First Exemplary Embodiment
In the printer U according to the first exemplary embodiment having
the above-described configuration, when an image is to be recorded
onto a recording sheet S, the second-transfer unit T2 receives a
second-transfer voltage Va. Specifically, in the first exemplary
embodiment, the second-transfer voltage Va is applied to the backup
roller T2a via the contact roller T2c. Thus, a transfer electric
field in accordance with the second-transfer voltage Va is
generated between the intermediate transfer belt B supported by the
backup roller T2a and the second-transfer roller T2b. Therefore,
when a visible image on the intermediate transfer belt B passes
through the nip region 16 between the intermediate transfer belt B
and the second-transfer roller T2b, the transfer electric field
acts on the visible image. Thus, the visible image is transferred
from the intermediate transfer belt B onto the recording sheet S.
In the first exemplary embodiment, the time constants .tau.s and
.tau.v of the second-transfer roller T2b and so on are set so as to
satisfy the relationships expressed by expression (11) and
expression (12).
FIGS. 9A and 9B illustrate a facing region in the image forming
apparatus. Specifically, FIG. 9A corresponds to FIG. 3, and FIG. 9B
is a cross-sectional view taken along line IXB-IXB in FIG. 9A.
Referring to FIGS. 9A and 9B, in an image forming apparatus, a nip
region 01 in the second-transfer region Q4 is normally given a
length, in the front-rear direction, based on the size of the
recording sheet S, that is, the size of the largest recording sheet
S on which an image is to be recorded. Therefore, if the recording
sheet S is not of the largest size, when the recording sheet S
passes through the nip region 01, a passing section 02 through
which the recording sheet S passes and a non-passing section 03
through which the recording sheet S does not pass occur in the nip
region 01. If an image is to be recorded after such passing section
02 and non-passing section 03 occur multiple times, an image defect
may possibly occur on a large-size recording sheet S. Specifically,
the resistance value of the intermediate transfer belt B is known
to decrease in the non-passing section 03. Thus, when recording an
image onto a large-size recording sheet, the transfer electric
field varies in the axial direction, causing an image defect, such
as a decrease in density and scattering of toner, to occur.
A decrease in resistance value of the intermediate transfer belt B
is caused by electric discharge occurring between the intermediate
transfer belt B and the second-transfer roller T2b. Specifically,
it is assumed that, when electric discharge occurs, the insulating
properties of the resin are lost. As a result, a conductive path
through which electricity travels easily is formed, causing the
resistance to decrease. Therefore, it is assumed that, when the
second-transfer voltage Va is high, the resistance tends to
decrease because electric discharge increases due to an increase in
potential difference between the intermediate transfer belt B and
the second-transfer roller T2b.
Therefore, in order to suppress a decrease in resistance of the
intermediate transfer belt B, it is conceivable that electric
discharge has to be controlled and suppressed.
After further researching on control and suppression of electric
discharge, it is conceivable that this electric discharge occurs
due to variations in microscopical spaces in the
electrical-conductivity additives 12 and 14 in the surface of the
second-transfer roller T2b. Specifically, in the roller layer 7 of
the second-transfer roller T2b, the electrical-conductivity
additive 14 is blended in the surface layer 9. Thus, when the
surface of the second-transfer roller T2b is viewed
microscopically, it may be considered that the
electrical-conductivity additive 14 having a small resistance value
is scattered throughout the resin 13 having a large resistance
value. Therefore, when the surface of the second-transfer roller
T2b is viewed microscopically, the surface of the second-transfer
roller T2b repeatedly has areas with a large resistance value and
areas with a small resistance value. The accumulability and the
movability of electric charge vary depending on the repeating cycle
of these areas, that is, a microscopical spatial distance between
resistance values according to the distance between the portions of
the electrical-conductivity additive 14, thus affecting the
electric discharge. A region in which the electrical-conductivity
additive 14 is sparsely distributed has a large amount of resin 13,
which has large resistance. In such a region, the aforementioned
spatial distance is long. In contrast, in a region in which the
electrical-conductivity additive 14 is densely distributed, the
portions of the electrical-conductivity additive 14 are close to
each other, so that the spatial distance is short.
Specifically, when voltage is applied between the intermediate
transfer belt B and the second-transfer roller T2b, electric
current flowing through the surface of the second-transfer roller
T2b tends to flow toward the electrical-conductivity additive 14
having a small resistance value rather than through the resin 13
having a large resistance value. In other words, in the
electrical-conductivity additive 14, electric charge readily moves
therethrough and readily accumulates therein. Thus, when the
transfer electric field becomes larger and electric discharge
occurs, the electric discharge tends to occur between the
electrical-conductivity additive 14 and the intermediate transfer
belt B. In this case, in the intermediate transfer belt B, it is
assumed that the electric discharge occurs near the
electrical-conductivity additive 14. Therefore, if the spatial
distance is long, since there are a small number of portions of the
electrical-conductivity additive 14, it is considered that areas
where electric discharge occurs tend to occur intensively also in
the intermediate transfer belt B. Thus, in order to alleviate a
decrease in resistance of the intermediate transfer belt B, the
electric discharge may conceivably be spread by increasing
microscopical points where the electric discharge occurs.
When an attempt to spread the electric discharge is performed by
shortening the spatial distance near the surface of the
second-transfer roller T2b by, for example, adjusting the blending
quantities of the electrical-conductivity additives 12 and 14,
concentration of the electric discharge in one area of the
intermediate transfer belt B may sometimes be largely reduced.
Specifically, it is discovered that, by adjusting the blending
quantities of the electrical-conductivity additives 12 and 14, a
decrease in resistance of the intermediate transfer belt B may be
suppressed.
In this case, with regard to the electric discharge occurring from
the electrical-conductivity additives 12 and 14 as points, the ease
of occurrence thereof may vary depending on the sizes, the
resistance values, the shapes, and so on of the
electrical-conductivity additives 12 and 14. In other words, a
minimal spatial distance for suppressing concentration of electric
discharge may vary depending on the types of
electrical-conductivity additives 12 and 14. In contrast, the
present inventor has discovered that concentration of electric
discharge in the second-transfer roller T2b may be suppressed by
causing the time constant .tau.s in the surface direction and the
time constant .tau.v in the volume direction to satisfy the
relationship expressed by expression (11), regardless of the types
of electrical-conductivity additives 12 and 14.
Specifically, in the first exemplary embodiment in which the time
constant .tau.s in the surface direction of the second-transfer
roller T2b is smaller than the time constant .tau.v in the volume
direction, concentration of electric discharge is reduced.
Therefore, in the first exemplary embodiment, a decrease in
resistance of the intermediate transfer belt B is also suppressed.
Thus, even when forming images onto recording sheets S of different
sizes, the occurrence of an image defect on a large-size recording
sheet S is reduced.
If the relationship expressed by expression (11) is not satisfied,
that is, if the time constant .tau.s in the surface direction is
larger than the time constant .tau.v in the volume direction, when
electric current flows between the shaft 6 and the outer surface 9a
of the second-transfer roller T2b, the electric current is less
likely to flow along the outer surface 9a. In other words, when
.tau.s>.tau.v, electric charge tends to be limited to moving in
one area of the outer surface 9a so that the transfer electric
field is generable only in one area, whereby the electric discharge
is less likely to spread.
FIGS. 10A to 10C illustrate distribution of the
electrical-conductivity additive. Specifically, FIG. 10A
corresponds to FIG. 4B, FIG. 10B is a comparative diagram, and FIG.
10C is a comparative diagram different from FIG. 10B.
Expression (11) will be complemented here. Including a large amount
of electrical-conductivity additive near the surface of a transfer
roller is equivalent to, for example, including a large amount of
electrical-conductivity additive 14 in the surface layer 9 in the
case of the second-transfer roller T2b having a double-layer
structure. In this case, the volume resistance value of the surface
layer 9 and the surface resistance value of the second-transfer
roller T2b decrease. However, for example, when carbon black 14' is
used as the electrical-conductivity additive 14, the spatial
distance varies as shown in FIGS. 10A to 10C even if the number of
particles of carbon black 14' is the same.
For example, in the surface layer 9 shown in FIG. 10A, the carbon
black 14' is distributed throughout the resin 13 with low
unevenness, that is, in a uniform manner. Specifically, with regard
to the distance between the particles of carbon black 14', there is
little variation in a distance d1 in the volume direction extending
from the shaft 6 toward the outer surface 9a. Furthermore, there is
little variation in a distance d2 in the circumferential direction
extending along the outer surface 9a. In FIG. 10A, with regard to
the distance between the particles of carbon black 14' in the layer
9, the distance d2 in the circumferential direction is averagely
shorter than the distance d1 in the volume direction.
The surface layer 9 shown in FIG. 10B repeatedly has, in the
circumferential direction, dense areas 13a in which the carbon
black 14' is densely distributed in the volume direction and
non-dense areas 13b in which the carbon black 14' does not exist.
Specifically, in the surface layer 9 shown in FIG. 10B, there is
little variation with regard to the distance d1 in the volume
direction. However, with regard to the distance d2 in the
circumferential direction, the distance d2 is small in the dense
areas, whereas the distance d2 is large in the non-dense areas.
Thus, the distance d2 in the circumferential direction varies
greatly and is nonuniform. In the surface layer 9 shown in FIG.
10C, the carbon black 14' is entirely lopsidedly distributed toward
the inner surface 9b. Some of the carbon black 14' is clustered
near the outer surface 9a. In this case, the clustered areas near
the outer surface 9a are distant from each other in the
circumferential direction. Therefore, in the surface layer 9 shown
in FIG. 10C, the distances d1 and d2 between the particles of
carbon black 14' vary greatly and are nonuniform. Thus, in the
surface layer 9 shown in each of FIGS. 10B and 10C, the distance d2
between the particles of carbon black 14' in the circumferential
direction near the outer surface 9a varies greatly and is
nonuniform.
A case where electric discharge occurs will now be discussed. In
the second-transfer roller T2b shown in FIG. 10A, the spatial
distance of the carbon black 14' is small, making it easier for the
electric discharge to spread since the electric discharge is less
likely to concentrate in one area. Thus, electric-discharge energy
per electrical conductive spot may spread readily. In contrast, in
the second-transfer roller T2b shown in each of FIGS. 10B and 10C,
the electric discharge tends to concentrate in the dense areas of
the carbon black 14' near the outer surface 9a. Thus, the
electric-discharge energy per electrical conductive spot tends to
increase. If the distribution of the carbon black 14' is uniform in
the circumferential direction, the electric discharge tends to
spread with decreasing distance d2.
Therefore, simply making the surface resistance value smaller than
the volume resistance value (surface resistance value<volume
resistance value) or making the surface resistivity smaller than
the volume resistivity (surface resistivity<volume resistivity)
by increasing the blending quantity of an electrical-conductivity
additive results in a transfer roller having areas with a large
spatial distance as in FIG. 10B or 10C, possibly resulting in a
situation where electric discharge between the intermediate
transfer belt B and the second-transfer roller T2b is not
alleviated. This may result in a high possibility of a decrease in
resistance of the intermediate transfer belt B.
In contrast, in the first exemplary embodiment in which expression
(11) is satisfied, the time constant .tau.s is smaller than the
time constant .tau.v. Thus, the spatial distance between the
portions of the electrical-conductivity additive 14 near the outer
surface 9a of the second-transfer roller T2b is maintained at a
certain value or smaller. Therefore, the configuration is limited
to a transfer roller with a small spatial distance, so that
concentration of electric discharge is alleviated. Consequently, in
the second-transfer roller T2b according to the first exemplary
embodiment, concentration of electric discharge may be readily
alleviated and a decrease in resistance of the intermediate
transfer belt B may be readily suppressed, as compared with the
configuration in the related art.
In the configuration in the related art, the cross section of the
roller layer 7 has to be observed by disassembling the
second-transfer roller T2b so as to determine whether or not the
spatial distance is small enough for alleviating electric
discharge. In other words, in the related art, the positional
relationship between the portions of the electrical-conductivity
additive 14 has to be observed. In contrast, in the first exemplary
embodiment, the relationship .tau.s<.tau.v is satisfied so that
the spatial distance of the electrical-conductivity additive 14 is
determined to be small without having to actually observe the cross
section of the roller layer 7. In other words, based on the
relationship .tau.s<.tau.v, the arrangement of the
electrical-conductivity additive 14 in the volume direction and the
surface direction is controlled so that the spatial distance of the
electrical-conductivity additive 14 is made small enough for
alleviating electric discharge.
FIGS. 11A and 11B illustrate uniform distribution of an
electrical-conductivity additive. Specifically, FIG. 11A
schematically illustrates a measurement method, and FIG. 11B
illustrates a measurement-result determination method.
With regard to a case where there is little variation in the
distribution of the electrical-conductivity additive 14 or 14', the
reason for uniformly distributing the electrical-conductivity
additive in the circumferential direction as shown in FIG. 10A will
be described in particular. In this specification, the uniform
distribution of the electrical-conductivity additive 14 or 14' in
the circumferential direction will be defined by using a standard
deviation .sigma. related to the time constant .tau.s of the
transfer roller. Specifically, in FIG. 11A, the time constant
.tau.s of the second-transfer roller T2b is measured at different
points P1 to P8 located at 45.degree. intervals in the
circumferential direction. In this specification, a state where the
standard deviation .sigma. with respect to the eight measured time
constants .tau.s is smaller than 1.0 will be defined as uniform
distribution of the electrical-conductivity additive 14 or 14' in
the circumferential direction. Thus, for example, referring to FIG.
11B, assuming that the time constants .tau.s are measured at the
positions P1 to P8 for each of samples 1 to 10 of second-transfer
rollers T2b, the samples 4, 7, 9, and 10 in which the standard
deviation .sigma. satisfies the condition .sigma.<1.0 are
regarded that the electrical-conductivity additive 14 is uniformly
distributed therein.
FIGS. 12A to 12D illustrate a distance in the volume direction and
a distance in the circumferential direction between portions of an
electrical-conductivity additive. Specifically, FIG. 12A
schematically illustrates a measurement method, FIG. 12B
illustrates a measurement result corresponding to FIG. 10A, FIG.
12C illustrates a measurement result corresponding to FIG. 10B, and
FIG. 12D illustrates a measurement result corresponding to FIG.
10C.
With regard to the distances between the portions of the
electrical-conductivity additive used in the above description, the
magnitude relationship between the distance d1 in the volume
direction and the distance d2 in the circumferential direction will
be described. In this specification, the distances d1 and d2 are
defined by using resistance values Rv and Rs measured for one
perimeter of the second-transfer roller T2b. Specifically, in FIGS.
12A to 12D, for the distance d1 in the volume direction, a volume
resistance value Rv for one perimeter of the second-transfer roller
T2b is measured. A difference .DELTA.Rv (=Rv1-Rv2) between a
maximum value Rv1 and a minimum value Rv2 of the measured
resistance value Rv expresses the aforementioned distance d1. For
the distance d2 in the circumferential direction, a surface
resistance value Rs for one perimeter of the second-transfer roller
T2b is measured. A difference .DELTA.Rs (=Rs1-Rs2) between a
maximum value Rs1 and a minimum value Rs2 of the measured
resistance value Rs expresses the aforementioned distance d2. Thus,
in the second-transfer roller T2b in which .DELTA.Rs<.DELTA.Rv
is satisfied, it is regarded that each of the
electrical-conductivity additives 12 and 14 is distributed in the
roller layer 7 such that the distance d2 between the portions of
the electrical-conductivity additive in the circumferential
direction of the outer surface 9a is shorter than the distance d1
between the portions of the electrical-conductivity additive in the
volume direction.
Normally, a resistance value of a transfer roller is dependent on
voltage. This dependency on voltage is classifiable into two types,
that is, an electronic conductive type and an ionic conductive
type, from the inclination of a resistance value relative to
applied voltage. An electronic conductive type is a type in which
an electronic conductive agent typified by carbon black carries
electric current, and has high voltage dependency. On the other
hand, an ionic conductive type is a type in which ions carry
electric current, and has low voltage dependency.
In transfer rollers in the related art, ionic conduction is
dominant, and the volume resistance value often decreases gradually
with increasing applied voltage. In this case, if the blending
quantity of carbon black near the surface is to be increased for
decreasing the surface resistance value, electronic conduction
becomes dominant over ionic conduction near the surface. Thus, the
surface resistance value has higher voltage dependency and
decreases sharply with increasing voltage. In other words, the
surface resistance value increases sharply with decreasing
voltage.
Normally, a resistance value of the second-transfer roller T2b is
measured with a voltage applied during a transfer process, such as
1000 V. Therefore, in the configuration in the related art, with
regard to a volume resistance value and a surface resistance value
measured at 1000 V, the surface resistance value is set to be
smaller than the volume resistance value. However, if the surface
resistance value is made smaller by increasing the blending
quantity of carbon black, the surface resistance value would
increase sharply with decreasing voltage, as described above. Thus,
at the low voltage side, the surface resistance value becomes
larger than the volume resistance value. Electric discharge occurs
when the potential difference with respect to the transfer roller
is about 300 V. Thus, in the case of the transfer roller in the
related art in which the surface resistance value is set to be
smaller than the volume resistance value, the surface resistance
value becomes larger than the volume resistance value in a low
voltage region of about 300 V, making it difficult to alleviate
concentration of electric discharge. Specifically, in the
configuration in the related art, expression (11) is not satisfied,
resulting in .tau.s>.tau.v. Since the aforementioned resistance
value may be read as resistivity, the condition .rho.s<.rho.v in
Japanese Unexamined Patent Application Publication No. 3-100579
generally results in .tau.s>.tau.v.
To describe this briefly, the idea of simply reducing the surface
resistance value or the surface resistivity as in the related art
only leads to an increase in the blending quantity of an
electrical-conductivity additive. This is equivalent to making the
electrical conducting mechanism into an electronic conductive type.
Thus, it is difficult to cope with the problem regarding electric
discharge occurring at the low voltage side. If carbon black is to
be increased so as to decrease the surface resistance value in a
transfer roller of an ionic conductive type, the behavior of
electronic conduction becomes stronger. Thus, it is extremely
difficult to decrease the resistance while maintaining ionic
conduction. Therefore, in either case, it is difficult to prevent
the surface resistance value from increasing sharply with
decreasing voltage.
In a double-layer structure including a base layer and a surface
layer, .tau.s<.tau.v may conceivably be achieved by largely
decreasing a resistance value of the surface layer. Specifically,
the surface resistance value may conceivably be decreased largely
in advance so that even when the surface resistance value increases
sharply with decreasing voltage, the surface resistance value is
smaller than the volume resistance value. However, in this state,
the transfer current does not flow to the shaft 6 but flows along
the surface of the second-transfer roller T2b to begin with. Thus,
before the occurrence of a problem of a decrease in resistance of
the non-passing section of the intermediate transfer belt B, a
problem of the transfer roller losing its function occurs.
In contrast, in the first exemplary embodiment, the second-transfer
roller T2b satisfies the condition .tau.s<.tau.v. Therefore, the
function of the transfer roller is ensured by adjusting the
relationship between the resistance values at about a voltage
applied during a transfer process, while the relationship between
the resistance values at about a voltage applied to the
second-transfer roller T2b during actual electric discharge between
the intermediate transfer belt B and the second-transfer roller T2b
is defined.
In the relationship expressed by expression (11), it is conceivable
that spreadability of electric discharge increases with decreasing
time constant .tau.s in the surface direction. Thus, in view of
suppressing concentration of electric discharge, it may seem it is
more desirable that the time constant .tau.s in the surface
direction be as small as possible. However, if the time constant
.tau.s in the surface direction is too small, a decrease in image
density may occur when recording an image onto, for example, thick
paper.
The present inventor has discovered that, when the time constant
.tau.s in the surface direction satisfies the relationship
expressed by expression (12), a transfer electric field may be
reliably ensured even when recording an image onto, for example,
thick paper. Thus, in the printer U according to the first
exemplary embodiment that satisfies the relationship expressed by
expression (12), a decrease in image density occurring with a
decrease in transfer electric field may be suppressed while an
image defect occurring with a decrease in resistance of the
intermediate transfer belt B may be suppressed.
Expression (12) will be complemented here:
(L/v).times.(Rv/Rs)<.tau.s<.tau.v (12)
In expression (12), (L/v) is in units of seconds and denotes a time
period from a point at which the outer surface 9a of the
second-transfer roller T2b enters the nip region 16 to a point at
which the outer surface 9a passes through the nip region 16, as
shown in FIG. 3.
Furthermore, (Rv/Rs) is a ratio between a resistance value
[.OMEGA.] and a resistance value [.OMEGA.] and denotes a
dimensionless value, that is, a coefficient.
In expression (12), (L/v) is equivalent to a time period during
which a certain position on the second-transfer roller T2b passes
through the nip region 16. Specifically, (L/v) indicates an
electric-potential rising period within the nip region 16, that is,
a transfer-electric-field rising period within the nip region 16.
Thus, although a transfer electric field for a second-transfer
process has to be generated within the passing time period (L/v),
the way in which the transfer electric field rises is dependent on
the resistance values of the second-transfer roller T2b. Therefore,
it is not desirable to randomly set a rotation speed v and the nip
width L. Specifically, the rotation speed v and the nip width L are
normally set by also taking into account a transfer voltage to be
applied in accordance with the resistance values of the
second-transfer roller T2b.
In expression (12), Rv denotes a volume resistance value and thus
has an effect on the transfer voltage.
If the passing time period (L/v) is short, the transfer electric
field has to rise rapidly. Therefore, Rv is set to a relatively
small value. In contrast, if the passing time period (L/v) is long,
the transfer electric field may rise gently. Therefore, Rv may be
set to a relatively large value.
By setting Rv to a relatively small value, the capacity of a
second-transfer power source may be reduced. This allows for use of
a low-voltage power source, thereby achieving lower cost. However,
this may lead to deterioration in image quality since there is a
large amount of electric discharge within the nip region 16. In
contrast, by setting Rv to a relatively large value, electric
discharge within the nip region 16 may be suppressed, thereby
achieving higher image quality. However, in this case, a
high-voltage power source may be necessary.
Consequently, the volume resistance value Rv is set in accordance
with the intended purpose.
Furthermore, Rv/Rs obtained by dividing the volume resistance value
Rv by the surface resistance value Rs increases with decreasing
surface resistance value Rs. This implies that flowability of
electric current in the surface direction of the second-transfer
roller T2b increases with decreasing surface resistance value Rs.
Thus, this implies that a current loss of electric current that
bypasses in the surface direction of the second-transfer roller
T2b, that is, a current loss of electric current less likely to
contribute to the transfer electric field, increases. Therefore,
(L/v).times.(Rv/Rs) in its entirety indicates the degree of current
loss in the surface direction of the second-transfer roller T2b
during the transfer-electric-field rising period (L/v).
In expression (12), the magnitude relationship between the time
constant .tau.s in the surface direction and the time constant
.tau.v in the volume direction is similar to that in expression
(11). Specifically, this implies that concentration of electric
discharge between the intermediate transfer belt B and the
second-transfer roller T2b is alleviated. In other words, when
.tau.s>.tau.v, the spatial distance of the
electrical-conductivity additive near the surface of the
second-transfer roller T2b is not sufficient for alleviating
concentration of electric discharge.
Second Exemplary Embodiment
Next, a second exemplary embodiment of the present invention will
be described. In the description of the second exemplary
embodiment, components that correspond to those in the first
exemplary embodiment are given the same reference characters, and
detailed descriptions thereof will be omitted.
The second exemplary embodiment differs from the first exemplary
embodiment in the following points but is similar to the first
exemplary embodiment in other points.
FIG. 13 is an enlarged view illustrating a relevant part of a
transfer member according to the second exemplary embodiment of the
present invention and corresponds to FIG. 4B in the first exemplary
embodiment.
In FIG. 13, in a roller layer 7' of the second-transfer roller T2b
according to the second exemplary embodiment, the
electrical-conductivity additives 12 and 14 are distributed more
densely toward the outer surface 9a from the shaft 6. Specifically,
the roller layer 7' in the second exemplary embodiment has the base
layer 8 similar to that in the first exemplary embodiment.
Furthermore, the outer surface 8a of the base layer 8 in the second
exemplary embodiment supports a surface layer 9' according to the
second exemplary embodiment in place of the surface layer 9
according to the first exemplary embodiment. In the surface layer
9' according to the second exemplary embodiment, the
electrical-conductivity additive 14 is distributed lopsidedly
toward the outer surface 9a. With regard to the
electrical-conductivity additive 14 distributed lopsidedly toward
the outer surface 9a, there is little variation in the distance d2
between the portions of the electrical-conductivity additive 14 in
the circumferential direction extending along the outer surface 9a.
Specifically, the electrical-conductivity additive 14 is uniformly
distributed in a state where there is little lopsidedness in the
circumferential direction.
Transfer-Member Manufacturing Method According to Second Exemplary
Embodiment
In the second exemplary embodiment, an electrode plate is disposed
facing the outer surface 8a of the base layer 8. Furthermore, in
the second exemplary embodiment, the resin liquid 43 is sprayed
onto the outer surface 8a of the base layer 8 while applying
voltage between the shaft 6 and the electrode plate. In other
words, in the second exemplary embodiment, an electric field that
causes the electrical-conductivity additive 32 within the resin
liquid 43 to move toward the outer surface 9a is generated. The
electric field is set in view of the movability of the
electrical-conductivity additive 32, such as the viscosity of the
resin liquid 43. Then, the electrical-conductivity additive 32 is
moved so as to be lopsided toward the outer surface 9a, whereby the
surface layer 9' is formed. The electrical-conductivity additive 32
may be preliminarily charged, or frictional electrification or the
like during feeding may be utilized. Alternatively, the
electrical-conductivity additive 32 may be lopsided by applying the
electric field during a drying and baking period after
spraying.
Operation of Second Exemplary Embodiment
In the second-transfer roller T2b according to the second exemplary
embodiment, expression (11) and expression (12) are satisfied.
Thus, the second exemplary embodiment is similar to the first
exemplary embodiment in that concentration of electric discharge
may be alleviated, and transferability onto thick paper may be
ensured.
In particular, in the surface layer 9' of the second-transfer
roller T2b according to the second exemplary embodiment, the
electrical-conductivity additive 14 is distributed lopsidedly
toward the outer surface 9a. In a configuration in which the
electrical-conductivity additive 14 is uniformly distributed
without any lopsidedness, if the number of portions of the
electrical-conductivity additive is to be increased in the surface
layer so as to satisfy expression (11), the volume resistance value
of the transfer roller tends to decrease. Thus, even if
concentration of electric discharge is alleviated in the
non-passing section by satisfying expression (11), there is a
possibility that electric discharge toward the toner in the passing
section within the nip region 16 may increase. In other words,
image quality may possibly deteriorate. In contrast, in the second
exemplary embodiment, expression (11) may be readily satisfied
without causing the volume resistance value to largely decrease.
Therefore, in the second exemplary embodiment, concentration of
electric discharge may be readily alleviated without causing
deterioration in image quality, and a decrease in resistance of the
intermediate transfer belt B may be readily suppressed, as compared
with a case where the electrical-conductivity additive is uniformly
distributed within the surface layer.
FIGS. 14A and 14B illustrate distribution of the
electrical-conductivity additive in accordance with the second
exemplary embodiment. Specifically, FIG. 14A corresponds to FIG.
13, and FIG. 14B is a comparative diagram in a case where the
electrical-conductivity additive is uniformly distributed.
Furthermore, for example, in FIG. 14B, in a case where the number
of portions of the electrical-conductivity additive in the surface
layer is small, if the electrical-conductivity additive 14 is
uniformly distributed without any lopsidedness, expression (11) may
sometimes be not satisfied. Specifically, in a configuration in
which a small number of portions of the electrical-conductivity
additive are uniformly distributed, the time constant .tau.s
becomes larger than the time constant .tau.v, resulting in a large
spatial distance. In this case, in the second exemplary embodiment
in which the electrical-conductivity additive 14 is distributed
lopsidedly toward the outer surface 9a, the number of portions of
the electrical-conductivity additive 14 is the same as that in FIG.
14B, and the time constant .tau.s may become smaller than the time
constant .tau.v even if the surface resistance value and the volume
resistance value are not different from those in FIG. 14B.
Therefore, in the lopsided configuration as in the second exemplary
embodiment, concentration of electric discharge may readily be
alleviated and a decrease in resistance of the intermediate
transfer belt B may be readily suppressed with a smaller number of
portions of the electrical-conductivity additive, as compared with
a configuration in which the electrical-conductivity additive is
uniformly distributed in the entire layer.
Third Exemplary Embodiment
Next, a third exemplary embodiment of the present invention will be
described. In the description of the third exemplary embodiment,
components that correspond to those in the first and second
exemplary embodiments are given the same reference characters, and
detailed descriptions thereof will be omitted.
The third exemplary embodiment differs from the first and second
exemplary embodiments in the following points but is similar to the
first and second exemplary embodiments in other points.
Value Settings of Transfer Member According to Third Exemplary
Embodiment
With regard to the second-transfer roller T2b according to the
third exemplary embodiment, a second-transfer roller T2b that
satisfies the relationship expression by expression (21) shown
below in place of expression (12) is used. Specifically, in the
third exemplary embodiment, when an Asker C hardness of the outer
surface 9a of the roller layer 7 of the second-transfer roller T2b
is defined as H, the time constant .tau.s [s] in the surface
direction and the time constant .tau.v [s] in the volume direction
satisfy the relationship expressed by expression (21) shown below:
(1/H).times.0.5<.tau.s<.tau.v (21)
FIG. 15 illustrates the relationship between the nip width of the
second-transfer roller and the hardness of the second-transfer
roller.
Normally, the transfer pressure in the second-transfer region Q4 is
set in advance. Load is applied onto the second-transfer roller T2b
in accordance with a hardness H2 of the second-transfer roller T2b
so that the transfer pressure is achieved. In this case, expression
(22) shown below stands between the hardness H (=H2) of the
second-transfer roller T2b and the width L of the nip region 16 in
the second-transfer region Q4 relative to an
experimentally-determined coefficient Z: L=Z/H (22)
In FIG. 15, for example, when the roller length .lamda.3 of the
second-transfer roller T2b in the axial direction is 320 mm and the
transfer pressure is about 4.3 N/cm.sup.2, experimentally-based
expression (22') shown below stands between H and L: L=125/H
(22')
In the aforementioned second-transfer roller T2b, when the hardness
H is 25 degrees or 40 degrees, foamed rubber is used for the base
layer 8. If the hardness H is 75 degrees, solid rubber is used for
the base layer 8. Furthermore, in the second-transfer roller T2b,
in order to maintain the transfer pressure at 4.3 N/cm.sup.2, 68 N
is set when the transfer-roller hardness is 25 degrees, 47 N is set
when the transfer-roller hardness is 40 degrees, and 25 N is set
when the transfer-roller hardness is 75 degrees.
By incorporating expression (22) into expression (12) in the first
exemplary embodiment and reorganizing the leftmost side of
expression (12), expression (23) shown below is obtained:
.times..times..times..times..times..times. ##EQU00003##
Thus, expression (12) is transformable into expression (24) shown
below by using expression (23):
(1/H).times.(Z/v).times.(Rv/Rs)<.tau.s<.tau.v (24)
Next, (Z/v).times.(Rv/Rs), which is a coefficient part of (1/H) in
expression (24), will be described.
By substituting A for (Z/v).times.(Rv/Rs), a coefficient A of (1/H)
is estimated.
First, before estimating the coefficient A, the physical meaning of
expression shown above will be confirmed. When the rotation speed v
is low, Rv/Rs is set to a small value. This implies that, the lower
the rotation speed v, the smaller the threshold value for a current
loss in the surface direction of the second-transfer roller T2b. In
other words, Rs may be set to a smaller value as the rotation speed
v increases, so that Rv/Rs is set to a large value. Specifically,
this implies that a current loss in the surface direction of the
second-transfer roller T2b tends to occur more with decreasing
rotation speed.
Next, expression (24) is transformed. A time constant of a
dielectric member is normally determined based on the resistance
value and the electrostatic capacitance of the dielectric member.
Specifically, the time constant .tau.s in the surface direction may
be considered as a product of the surface resistance value Rs and
an electrostatic capacitance Cs in the surface direction. The
electrostatic capacitance Cs in the surface direction is an
electrostatic capacitance of a surface section of the
second-transfer roller T2b. Therefore, by substituting
.tau.s=Rs.times.Cs for .tau.s in expression (24), expression (25)
shown below with respect to the relationship between the left side
of expression (24) and the time constant .tau.s in the surface
direction is obtained:
(1/H).times.(Z/v).times.(Rv/Rc)<Cs.times.Rs (25)
Consequently, expression (26) shown below is obtained from the
relationship between expression (25) and A:
(Z/v).times.(Rv/Rs)<H.times.Cs.times.Rs
A=A(v,Rv,Rs)<H.times.Cs.times.Rs (26)
Thus, A is set as a value that satisfies expression (26). In this
case, A(v,Rv,Rs) denotes that the coefficient A is a function of v,
Rv, and Rs. Then, a maximum value, that is, a threshold value, of A
that satisfies expression shown above (a random combination does
not satisfy expression shown above) when v, Rv, Rs, H, and Cs are
set is determined.
However, it is extremely difficult to analytically determine the
coefficient A. Thus, the coefficient A is experimentally estimated
based on several conditions. With regard to the experimental
conditions, suitably usable numerical values are used for the
second-transfer roller. In the third exemplary embodiment,
experiments are performed on a total of 12 second-transfer rollers
T2b. Specifically, when the surface resistance value Rs and the
volume resistance value Rv of each second-transfer roller T2b are
expressed as (Rs[log .OMEGA.], Rv[log .OMEGA.]), for example, the
second-transfer rollers T2b have six patterns of resistance values,
i.e., (7.8, 8.3), (8.1, 8.0), (8.1, 8.3), (8.3, 8.0), (8.3, 8.3),
and (8.3, 8.7), and two patterns of Asker C hardness, i.e., 25
degrees and 75 degrees. With regard to the hardness H of the
second-transfer roller T2b, an Asker C hardness ranging between 25
degrees and 75 degrees are suitably usable. Therefore, the boundary
values are used as the experimental conditions.
Furthermore, the electrostatic capacitance Cs in the surface
direction is estimated by measuring the impedance. Specifically,
the impedance is measured by using a dielectric-constant
measurement interface of model 1296 and an impedance analyzer of
model 1260, which are manufactured by Solartron Group Ltd. In this
case, the applied voltage is 1 V and 3 V based on alternating
current. Furthermore, the measurement frequencies of real and
imaginary parts are measured in conditions from 10 mHz to 1 MHz.
Then, by using a value obtained by fitting as an example of an
approximate-expression deriving technique, the electrostatic
capacitance Cs is estimated with a capacitor-resistor (CR) circuit
using analysis software based on the measurement value of each of
the real and imaginary parts. Evaluations are performed by setting
the rotation speed v to 440 mm/s and 600 mm/s.
As the volume resistance value of the backup roller T2a,
10.sup.7.0.OMEGA. is used. However, electric discharge between the
intermediate transfer belt B and the second-transfer roller T2b is
determined based on voltage applied to a so-called gap between the
intermediate transfer belt B and the second-transfer roller T2b.
Therefore, the energy during the electric discharge is dependent on
the electrical conductive spots of the second-transfer roller T2b,
that is, the spatial distance of the electrical-conductivity
additive. Thus, the volume resistance value of the backup roller
T2a substantially has no effect.
FIG. 16 illustrates evaluation results of the coefficient A.
FIG. 17 illustrates a maximum coefficient A that satisfies
expression (26) for each speed and each hardness.
In FIG. 16, when the coefficient A satisfies expression (26),
transferability onto small-size thick paper is satisfactory. Thus,
as shown in FIG. 16, when the coefficient A satisfies expression
(26), a cell corresponding to transferability onto small-size thick
paper is given a circle. A maximum coefficient A that satisfies
expression (26) for each hardness H and each speed v is shown in
FIG. 17. Specifically, in FIG. 17, a threshold value for the
coefficient A, which indicates that transferability onto small-size
thick paper is satisfactory when the coefficient A is smaller than
or equal to this value, is shown.
As a result, in FIGS. 16 and 17, it is confirmed that when the
coefficient A is smaller than or equal to 0.9, a transfer defect on
small-size thick paper may be suppressed. Specifically, it is
confirmed that when the hardness is 75 degrees and the rotation
speed v is 440 mm/s, a transfer defect may be suppressed even if
the coefficient A is 0.9. However, in a case of high-speed
rotation, that is, when the rotation speed v is 400 mm/s or higher,
high transfer voltage may generally be necessary. When the hardness
or the rotation speed changes, it is confirmed that the coefficient
A has to be further reduced from 0.9. It is confirmed that when the
coefficient A is smaller than or equal to 0.5 and is sufficiently
small, a transfer defect on small-size thick paper may be
suppressed even when the hardness or the rotation speed changes, as
shown in FIGS. 16 and 17.
Consequently, based on the above evaluation results, a coefficient
A that satisfies expression (27) shown below may be estimated:
A<0.5 (27)
Thus, by undoing A by dividing both sides of expression (27) by H,
expression (28) shown below is obtained:
(1/H).times.(Z/v).times.(Rv/Rs)<(1/H).times.0.5 (28)
In view of expression (24), expression (28), and the magnitude
relationship of .tau.s, .tau.v, and A in the evaluation results
shown in FIG. 16, expression (21) is obtained:
(1/H).times.0.5<.tau.s<.tau.v (21)
Operation of Third Exemplary Embodiment
In the second-transfer roller T2b according to the third exemplary
embodiment having the above-described configuration, the time
constant .tau.s in the surface direction and the time constant
.tau.v in the volume direction satisfy the relationship expressed
by expression (11). Thus, similar to the first exemplary
embodiment, concentration of electric discharge may be alleviated.
In particular, in the third exemplary embodiment, the time constant
.tau.s in the surface direction, the time constant .tau.v in the
volume direction, and the Asker C hardness H of the second-transfer
roller T2b satisfy the relationship expressed by expression (21).
Thus, in the printer U according to the third exemplary embodiment
that satisfies expression (21), even when an image is to be
recorded onto, for example, thick paper, a transfer electric field
may be readily ensured, as compared with a case where a lower limit
for the time constant .tau.s in the surface direction is not set.
Thus, in the printer U according to the third exemplary embodiment,
a decrease in image density occurring with a decrease in transfer
electric field may be suppressed while an image defect occurring
with a decrease in resistance of the intermediate transfer belt B
may be suppressed.
EXPERIMENTAL EXAMPLE
Next, experiments for checking the effects of the first to third
exemplary embodiments are performed.
Intermediate Transfer Body and Second-Transfer Unit According to
Experimental Example
Referring to FIGS. 1 to 3, the following configuration is used in
the experimental example.
With regard to the backup roller T2a, the shaft 1 has a diameter of
14 mm, the roller layer 2 has a thickness of 5 mm, the hardness H1
is an Asker C hardness of 60 degrees, and the volume resistance
value is 10.sup.7.0.OMEGA. at an applied voltage of 1 kV.
The intermediate transfer belt B is composed of polyimide with
carbon black blended therein. The intermediate transfer belt B
according to the experimental example has a thickness of 80 .mu.m,
a volume resistivity of 10.sup.10 .OMEGA.cm at an applied voltage
of 100 V, and a surface resistivity of 10.sup.10 .OMEGA./sq. at an
applied voltage of 100 V.
With regard to the second-transfer roller T2b, the shaft 6 has a
diameter of 14 mm, and the roller layer 7 has a double-layer
configuration in which the base layer 8 has a thickness of 5 mm and
the surface layer 9 has a thickness of 20 .mu.m. The length
.lamda.2 of the roller layer 7 according to the experimental
example in the front-rear direction is set to 320 mm. The volume
resistance value Rv and the surface resistance value Rs of the
second-transfer roller T2b according to the experimental example
are adjusted by independently controlling the resistance of the
base layer 8 and the resistance of the surface layer 9. A specific
configuration of the second-transfer roller T2b according to the
experimental example will be described later.
In the experimental example, Fuji Xerox J paper at 82 grams per
square meter, which is plain paper, is used as a recording sheet S
to be used for evaluation. As a recording sheet S of small-size
thick paper, a postcard is used.
An evaluation experiment is performed at a temperature of
10.degree. C. and a relative humidity of 15%.
Constant current control is performed by using a constant current
source as a second-transfer power source. When the transport speed
v is 528 mm/s, an electric current of 110 .mu.A is applied. When
the transport speed v is 264 mm/s, an electric current of 55 .mu.A
is applied.
For the second-transfer roller T2b, transfer load with a transfer
pressure of 4.3 N/cm.sup.2 is set. Specifically, when the hardness
of the transfer roller is 25 degrees, the transfer load is set to
68 N. When the hardness of the transfer roller is 40 degrees, the
transfer load is set to 47 N. When the hardness of the transfer
roller is 75 degrees, the transfer load is set to 25 N.
Transfer-Member Manufacturing Method According to Experimental
Example
Referring to FIGS. 5A and 5B, in the experimental example, a
specific configuration of the mixture 29 is as follows.
As the rubber material 21, epichlorohydrin rubber and
acrylonitrile-butadiene rubber, which have excellent ion
conductivity by containing an ethylene oxide group, are used.
Specifically, Epichlomer CG-102 manufactured by Daiso Co., Ltd. is
used as epichlorohydrin rubber. Moreover, Nipol DN-219 manufactured
by Zeon Corporation is used as acrylonitrile-butadiene rubber.
Furthermore, carbon black is used as the electrical-conductivity
additive 22. Specifically, Special Black 4A manufactured by Degussa
Corporation is used. The blending quantity of carbon black is
adjusted in accordance with the conditions of the second-transfer
roller to be formed. A description regarding the blending quantity
will be provided later.
Furthermore, sulfur is used as the vulcanizing agent 23.
Specifically, 200 mesh manufactured by Tsurumi Chemical Industry
Co., Ltd. is used.
Furthermore, Nocceler M manufactured by Ouchi Shinko Chemical
Industrial Co., Ltd. is used as the vulcanization accelerator
24.
The mixture 29 containing the above components is wrapped around
the shaft 6.
The shaft 6 having the mixture 29 wrapped therearound is increased
in temperature to 160.degree. C. and is vulcanized and foamed for a
predetermined time period, whereby a roller equipped with a base
layer is obtained.
Referring to FIGS. 6A to 6C, in the experimental example, a
specific configuration of the resin liquid 43 is as follows.
Butyl acetate is used as the solvent 36.
A tetrafluoroethylene-vinyl monomer copolymer is used as the
curable resin 33. Specifically, 100 parts of Zeffle GK-510
manufactured by Daikin Industries, Ltd. are used.
Carbon black is used as the electrical-conductivity additive 32.
Specifically, Special Black 4A manufactured by Degussa Corporation
is used. The blending quantity of carbon black is adjusted in
accordance with the conditions of the second-transfer roller to be
formed. A description regarding the blending quantity will be
provided later.
Geanus PY manufactured by Geanus Co., Ltd. is used as the jet-mill
distribution device 38.
With regard to the distribution process by the jet-mill
distribution device 38, a collision step is performed five times
under a pressure of 200 N/mm.sup.2.
As the mesh 39, 20-.mu.m mesh is used.
Takenate D-140N manufactured by Mitsui Chemicals, Inc. is used as
the curing agent 34. Specifically, 20 parts of Takenate D-140N
relative to 100 parts of Zeffle GK-510 in the base-layer resin
liquid 43 are used.
Referring to FIGS. 6A to 6C, the outer surface 8a of the base layer
8 around the shaft 6 is coated with a layer of the resin liquid 43
and is baked by being heated at 140.degree. C. for 20 minutes,
whereby the second-transfer roller T2b is formed.
Transfer-Member Measurement Method According to Experimental
Example
Referring to FIG. 7A, the length .lamda.3 of each of the metallic
plates 61 and 62 in the front-rear direction is set to 330 mm. The
length .lamda.4 of each of the metallic plates 61 and 62 in the
thickness direction is set to 2 mm. The metallic plates 61 and 62
are pressed against the outer surface 9a of the roller layer 7 such
that they are engaged therewith by 0.2 mm.
The peripheral length .lamda.5 between the right surface 61b of the
first metallic plate 61 and the left surface 62b of the second
metallic plate 62 is set to 2 mm.
Referring to FIG. 7B, the voltage V0 to be applied by the
direct-current voltage source 64 is set to 1000 V.
Furthermore, the time T1 is set to 10 seconds.
Based on this configuration, the time constant .tau.s in the
surface direction is measured.
Referring to FIG. 8A, for measuring the time constant .tau.v in the
volume direction, the voltage application is stopped from the state
where 1000 V is applied by the direct-current voltage source
64.
Based on this configuration, the time constant .tau.v in the volume
direction is measured.
In the experimental example, the volume resistance value Rv
[.OMEGA.] of the second-transfer roller T2b is measured using the
following measurement method.
Specifically, the shaft 6 is pressed with a load of 6 kgf toward a
ground-connected metal plate so that the outer surface 9a of the
second-transfer roller T2b is pressed thereon. A metallic rod is
brought into contact with the outer surface 9a of the
second-transfer roller T2b in a state where the metallic rod is
engaged therewith by 0.2 mm. Then, a voltage of 1000 V is applied
to the metallic rod, and the shaft 6 is connected to ground. An
electric current I [A] flowing through the shaft 6 is measured.
Subsequently, the volume resistance value Rv is calculated and
measured based on Rv [.OMEGA.]=1000 [V]/I[A].
In the experimental example, the surface resistance value Rs
[.OMEGA.] of the second-transfer roller T2b is measured using the
following measurement method.
Specifically, the shaft 6 of the second-transfer roller T2b is
connected to ground, and two metallic rods are disposed on the
surface of the second-transfer roller T2b. The two metallic rods
each have a diameter of 12 mm and a length of 330 mm. The two
metallic rods are disposed away from each other by 10 mm in the
circumferential direction of the second-transfer roller T2b and are
brought into contact therewith in a state where they are engaged
therewith by 0.2 mm. Then, a voltage of 1000 V is applied to one of
the two metallic rods while the other metallic rod is connected to
ground. An electric current I [A] flowing through the other
metallic rod is measured. Subsequently, the surface resistance
value Rs is calculated and measured based on Rs [.OMEGA.]=1000
[V]/I [A].
Experimental Example 1-1
In an experimental example 1-1, the hardness H of the
second-transfer roller T2b is set to an Asker C hardness of 25
degrees. Furthermore, the relationship (1/H).times.0.5<.tau.s
and the relationship .tau.s<.tau.v are satisfied by adjusting
.tau.s, .tau.v, Rs, and Rv of the second-transfer roller T2b.
Then, in the experimental example 1-1, an evaluation experiment is
performed by using the second-transfer roller T2b. In the
evaluation method according to the experimental example 1-1,
various kinds of measurement and evaluation processes are performed
after successively feeding 50,000 sheets of J paper, which is
size-A3 evaluation paper, at a processing speed of 528 mm/s. Thus,
the rotation speed v of the second-transfer roller T2b corresponds
to 528 mm/s. In this case, with regard to the non-passing section
of the intermediate transfer belt B, the surface resistivity of a
surface facing the second-transfer roller T2b is measured. Then, it
is confirmed whether an amount of change in surface resistivity
from an initial state where there is no problem in image quality is
smaller than or equal to 0.20 log .OMEGA./sq. Furthermore, for
checking for a transfer defect caused by a current loss related to
the lower limit of the time constant .tau.s, a blue solid image is
printed on the entire face of a postcard having high sensitivity,
and transferability thereon is checked. In the experimental example
1-1, three parts of carbon black are blended in the base layer 8,
and three parts of carbon black are blended in the surface layer
9.
Experimental Example 1-2
In an experimental example 1-2, the hardness H of the
second-transfer roller T2b is set to an Asker C hardness of 75
degrees. Furthermore, the relationship .tau.s<.tau.v is
satisfied by adjusting .tau.s, .tau.v, Rs, and Rv of the
second-transfer roller T2b. However, in the experimental example
1-2, the relationship (1/H).times.0.5<.tau.s is not satisfied.
In the experimental example 1-2, six parts of carbon black are
blended in the base layer 8, and eight parts of carbon black are
blended in the surface layer 9. Other conditions and the evaluation
method are the same as those in the experimental example 1-1.
Experimental Example 1-3
In an experimental example 1-3, the hardness H of the
second-transfer roller T2b is set to an Asker C hardness of 75
degrees. Furthermore, the relationship (1/H).times.0.5<.tau.s
and the relationship .tau.s<.tau.v are satisfied by adjusting
.tau.s, .tau.v, Rs, and Rv of the second-transfer roller T2b. In
the experimental example 1-3, four parts of carbon black are
blended in the base layer 8, and five parts of carbon black are
blended in the surface layer 9. Other conditions and the evaluation
method are the same as those in the experimental example 1-1.
Comparative Example 1
In a comparative example 1, a second-transfer roller having a
configuration normally used in the related art is used. In the
second-transfer roller T2b according to the comparative example 1,
the hardness H is set to an Asker C hardness of 25 degrees.
Furthermore, in the second-transfer roller according to the
comparative example 1, the relationship (1/H).times.0.5<.tau.s
is satisfied. However, the relationship .tau.s<.tau.v is not
satisfied. In the comparative example 1, five parts of carbon black
are blended in the base layer 8, and three parts of carbon black
are blended in the surface layer 9. Other conditions and the
evaluation method are the same as those in the experimental example
1-1.
Comparative Example 2
In a comparative example 2, the hardness H of the second-transfer
roller T2b is set to an Asker C hardness of 75 degrees.
Furthermore, in the comparative example 2, Rs and Rv of the
second-transfer roller T2b are the same as Rs and Rv used in the
experimental example 1-1. However, .tau.s and .tau.v of the
second-transfer roller T2b according to the comparative example 2
do not satisfy the relationship .tau.s<.tau.v. Moreover, .tau.s
and .tau.v in the comparative example 2 satisfy the relationship
(1/H).times.0.5<.tau.s. In the second-transfer roller T2b
according to the comparative example 2, four parts of carbon black
are blended in the base layer 8, and four parts of carbon black are
blended in the surface layer 9. Other conditions and the evaluation
method are the same as those in the experimental example 1-1.
Experimental Results of Experimental Examples 1-1 to 1-3 and
Comparative Examples 1 and 2
FIG. 18 illustrates conditions and experimental results of the
experimental example 1-1, the experimental example 1-2, the
experimental example 1-3, the comparative example 1, and the
comparative example 2.
Referring to FIG. 18, in the experimental examples 1-1 to 1-3 that
satisfy the relationship .tau.s<.tau.v, it is confirmed that the
surface resistivity of the intermediate transfer belt B has not
decreased. On the other hand, in the comparative examples 1 and 2
that do not satisfy the relationship .tau.s<.tau.v, it is
confirmed that the surface resistivity of the intermediate transfer
belt B has decreased. Thus, it is confirmed that a decrease in
resistance in the non-passing section of the intermediate transfer
belt B may be suppressed and that a change in density in the
non-passing section may be suppressed regardless of the hardness H
of the transfer roller so long as the transfer roller satisfies the
relationship .tau.s<.tau.v, that is, expression (11). In other
words, it is confirmed that concentration of electric discharge may
be suppressed by satisfying expression (11).
The resistance values Rv and Rs are the same between the
experimental example 1-1 and the comparative example 2. However, a
decrease in resistance of the intermediate transfer belt B is not
confirmable in the experimental example 1-1. In contrast, a
decrease in resistance of the intermediate transfer belt B has
occurred in the comparative example 2. Thus, it is confirmed that
it is conceivably difficult to determine whether or not
concentration of electric discharge is alleviated based only on the
resistance values Rv and Rs of the second-transfer roller T2b. In
other words, in the first to third exemplary embodiments in which
definition is made based on the relationship between the time
constants .tau.s and .tau.v, it is possible to accurately evaluate
whether or not concentration of electric discharge may be
alleviated, unlike the related art in which a value related to a
resistance value, such as resistivity, is used.
Furthermore, in the experimental examples 1-1 and 1-3 that satisfy
the relationship (1/H).times.0.5<.tau.s when .tau.s<.tau.v,
it is confirmed that a transfer defect on a postcard does not
occur. On the other hand, in the experimental example 1-2 that does
not satisfy the relationship (1/H).times.0.5<.tau.s when
.tau.s<.tau.v or the comparative example 1 in which
.tau.s>.tau.v, it is confirmed that a transfer defect on a
postcard does occur. Thus, it is confirmed that, by satisfying the
relationship (1/H).times.0.5<.tau.s, that is, the relationship
expressed by expression (21), a decrease in resistance in the
non-passing section of the intermediate transfer belt B may be
suppressed and a change in density in the non-passing section may
be suppressed while ensuring satisfactory transferability onto
small-size thick paper.
Experimental Example 2-1
In an experimental example 2-1, the values of H, .tau.s, .tau.v,
Rs, and Rv are the same as those of the second-transfer roller T2b
according to the experimental example 1-1. Thus, the relationship
expressed by expression (11) is satisfied. In the experimental
example 2-1, the hardness H is 25 degrees, and the nip width is 5.0
mm in accordance with expression (22').
In the experimental example 2-1, an evaluation experiment is
performed by using the second-transfer roller T2b having the
above-described configuration. In the evaluation method according
to the experimental example 2-1, the evaluation experiment is
performed similarly to the experimental example 1-1 except that
transferability onto small-size thick paper is evaluated in view of
the effect of the processing speed, that is, the effect of the
transfer-electric-field rising period. In the experimental example
2-1, the processing speed v is 528 mm/s, and the relationship
(L/v).times.(Rv/Rs)<.tau.s is satisfied.
Experimental Example 2-2
In an experimental example 2-2, the values of H, .tau.s, .tau.v,
Rs, and Rv are the same as those of the second-transfer roller T2b
according to the experimental example 1-2. Thus, the relationship
expressed by expression (11) is satisfied. In the experimental
example 2-2, the hardness H is 75 degrees, and the nip width is 1.7
mm. Other conditions and the evaluation method are the same as
those in the experimental example 2-1. In the experimental example
2-2, the processing speed v is 528 mm/s, and the relationship
(L/v).times.(Rv/Rs)<.tau.s is not satisfied.
Experimental Example 2-3
In an experimental example 2-3, the values of H, .tau.s, .tau.v,
Rs, and Rv are the same as those of the second-transfer roller T2b
according to the experimental example 1-3. Thus, the relationship
expressed by expression (11) is satisfied. In the experimental
example 2-3, the hardness H is 75 degrees, and the nip width is 1.7
mm. Other conditions and the evaluation method are the same as
those in the experimental example 2-1. In the experimental example
2-3, the processing speed v is 528 mm/s, and the relationship
(L/v).times.(Rv/Rs)<.tau.s is satisfied.
Experimental Example 2-4
In an experimental example 2-4, the relationship
(L/v).times.(Rv/Rs)<.tau.s<.tau.v is satisfied under the
pressing speed v of 264 mm/s by adjusting .tau.s, .tau.v, Rs, and
Rv of the second-transfer roller T2b. Other conditions and the
evaluation method are the same as those in the experimental example
2-3.
Experimental Example 2-5
In an experimental example 2-5, the values of H, .tau.s, .tau.v,
Rs, and Rv are the same as those of the second-transfer roller T2b
according to the experimental example 2-3. Thus, the relationship
expressed by expression (11) is satisfied. However, in the
experimental example 2-5, the width L of the nip region is set to
1.3 mm by weakening the transfer load. Furthermore, in the
experimental example 2-5, the processing speed v is 264 mm/s.
Specifically, in the experimental example 2-5, of L, v, Rv, Rs, and
.tau.s in the experimental example 2-3 related to
(L/v).times.(Rv/Rs) and .tau.s, the relationship
(L/v).times.(Rv/Rs)<.tau.s is satisfied by changing L and v.
Other conditions and the evaluation method are the same as those in
the experimental example 2-3.
Experimental Results of Experimental Examples 2-1 to 2-5
FIG. 19 illustrates conditions and experimental results of the
experimental example 2-1, the experimental example 2-2, the
experimental example 2-3, the experimental example 2-4, and the
experimental example 2-5.
Referring to FIG. 19, in the experimental examples 2-1 to 2-5, the
relationship expressed by expression (11) is satisfied. In the
experimental examples 2-1 to 2-5, it is confirmed that the surface
resistivity of the intermediate transfer belt B is less likely to
decrease. Thus, it is reconfirmed that concentration of electric
discharge may be suppressed by satisfying expression (11).
Furthermore, in the experimental examples 2-1, 2-3, 2-4, and 2-5
that satisfy the relationship
(L/v).times.(Rv/Rs)<.tau.s<.tau.v, it is confirmed that a
transfer defect on a postcard does not occur. On the other hand, in
the experimental example 2-2 that does not satisfy the relationship
(L/v).times.(Rv/Rs)<.tau.s, it is confirmed that a transfer
defect on a postcard does occur. Thus, it is confirmed that, by
satisfying the relationship
(L/v).times.(Rv/Rs)<.tau.s<.tau.v, that is, the relationship
expressed by expression (12), a decrease in resistance in the
non-passing section of the intermediate transfer belt B may be
suppressed and a change in density in the non-passing section may
be suppressed while ensuring satisfactory transferability onto
small-size thick paper.
Fourth Exemplary Embodiment
Next, a fourth exemplary embodiment of the present invention will
be described. In the description of the fourth exemplary
embodiment, components that correspond to those in the first to
third exemplary embodiments are given the same reference
characters, and detailed descriptions thereof will be omitted.
The fourth exemplary embodiment differs from the first exemplary
embodiment in the following points but is similar to the first
exemplary embodiment in other points.
FIG. 20 illustrates a relevant part of a transfer device according
to the fourth exemplary embodiment of the present invention.
Referring to FIG. 20, the second-transfer unit T2 as an example of
a transfer device according to the fourth exemplary embodiment has
a second-transfer roller T2b similar to that in the first exemplary
embodiment. Specifically, with regard to the second-transfer roller
T2b according to the fourth exemplary embodiment, the time constant
.tau.s in the surface direction and the time constant .tau.v in the
volume direction are set such that .tau.s<.tau.v. Furthermore,
in the fourth exemplary embodiment, the outer surface 9a of the
second-transfer roller T2b is formed to have a predetermined
surface roughness Rz. The surface roughness Rz, that is, a
ten-point medium height Rz, is desirably 2.0 .mu.m or smaller. The
contact roller T2c is connected to a power source E1. The power
source E1 according to the fourth exemplary embodiment only applies
voltage with a polarity for transferring a visible image on the
intermediate transfer belt B onto a recording sheet S.
Specifically, the power source E1 according to the fourth exemplary
embodiment only applies voltage with the same polarity as the
charge polarity of toner Tn as an example of a developer to the
backup roller T2a via the contact roller T2c. The shaft 6 of the
second-transfer roller T2b is electrically connected to ground.
A brush device 101 as an example of a first cleaning device is
disposed downstream of the second-transfer region Q4 in the
rotational direction of the second-transfer roller T2b. The brush
device 101 has a cleaning container 102. The cleaning container 102
rotatably supports an electrostatic brush 103 as an example of a
first electrically-conductive cleaning member. The electrostatic
brush 103 has a shaft 103a as an example of a rotation shaft. The
shaft 103a is composed of a metallic material as an example of an
electrically-conductive material. The shaft 103a is electrically
connected to ground. The outer peripheral surface of the shaft 103a
has multiple electrically-conductive bristles implanted therein at
a predetermined density. Specifically, the shaft 103a supports a
brush portion 103b as an example of a brush portion having multiple
electrically-conductive bristles extending radially therefrom. The
brush portion 103b comes into contact with the surface of the
second-transfer roller T2b at a cleaning position Q101 as an
example of a position where the brush portion 103b comes into
contact with the second-transfer roller T2b. The shaft 103a
receives a driving force from a driving source (not shown). Thus,
at the cleaning position Q101, the electrostatic brush 103 rotates
at a predetermined speed in a direction opposite to the rotational
direction of the second-transfer roller T2b.
In the brush device 101, a lubricant 104 is disposed downstream of
the cleaning position Q101 in the rotational direction of the
electrostatic brush 103. The lubricant 104 is supported by a bias
member 106. The bias member 106 biases the lubricant 104 with a
predetermined bias force such that the lubricant 104 comes into
contact with the brush portion 103b of the electrostatic brush 103.
Thus, the lubricant 104 is supplied to the surface of the
second-transfer roller T2b via the electrostatic brush 103. The
lubricant 104 may be composed of a solid material, such as zinc
stearate (ZnSt). The lubricant 104 and the bias member 106
constitute a lubricant supplying section 104+106 according to the
fourth exemplary embodiment. A flicker 107 as an example of an
adjusting member is disposed downstream of the lubricant 104 in the
rotational direction of the electrostatic brush 103. The flicker
107 is disposed in contact with the brush portion 103b. A discharge
transport member 108 is disposed below the electrostatic brush 103.
The developer collected by the electrostatic brush 103 from the
second-transfer roller T2b is transported toward a collecting
container (not shown) by the discharge transport member 108.
In FIG. 20, a blade device 111 as an example of a second cleaning
device is disposed downstream of the cleaning position Q101 in the
rotational direction of the second-transfer roller T2b. The blade
device 111 has a cleaning container 112. The cleaning container 112
supports a plate-shaped cleaning blade 113 as an example of a
second cleaning member. The cleaning blade 113 comes into contact
with the surface of the second-transfer roller T2b at a second
cleaning position Q102 as an example of a second contact position.
The cleaning blade 113 is in contact with the surface of the
second-transfer roller T2b with a predetermined pressure. A
discharge transport member 114 is disposed below the cleaning blade
113. The developer removed from the second-transfer roller T2b by
the cleaning blade 113 is transported toward a collecting container
(not shown) by the discharge transport member 114.
In the fourth exemplary embodiment, when the distance from a nip
exit Q103, as an example of a downstream end of the nip region in
the rotational direction of the second-transfer roller T2b, to the
cleaning position Q101 is defined as La [mm], La is set such that
expression (41) shown below is satisfied:
La.ltoreq.{(.tau.v/.tau.s)/1.25}.times.12.pi. (41)
In the fourth exemplary embodiment, when the nip region 16 is
formed by the backup roller T2a and the second-transfer roller T2b,
a position where the backup roller T2a and the second-transfer
roller T2b are less likely to receive pressure from each other is
set as the downstream end of the nip region 16, that is, the nip
exit Q103. Specifically, when a recording sheet S moves through the
nip region 16 in the transport direction thereof, a position where
a gap 121 forms between the outer surface 9a of the second-transfer
roller T2b and the outer surface of the backup roller T2a is
defined as the nip exit Q103.
Operation of Fourth Exemplary Embodiment
In the printer U according to the fourth exemplary embodiment
having the above-described configuration, when an image is to be
recorded onto a recording sheet S, the second-transfer unit T2
receives a second-transfer voltage from the power source E1. Thus,
a transfer electric field in accordance with the second-transfer
voltage is generated between the intermediate transfer belt B and
the second-transfer roller T2b. Therefore, the transfer electric
field acts on a visible image on the intermediate transfer belt B
so that the visible image becomes transferred from the intermediate
transfer belt B onto the recording sheet S. In the second-transfer
roller T2b according to the fourth exemplary embodiment, expression
(11) and expression (12) are satisfied. Therefore, the fourth
exemplary embodiment is similar to the first exemplary embodiment
in that concentration of electric discharge may be alleviated, and
transferability onto thick paper may be ensured.
The intermediate transfer belt B sometimes bears a developer Tn,
which constitutes a visible image, in the non-passing section of
the intermediate transfer belt B, through which a recording sheet S
does not pass, or in an area between a recording sheet S and a
recording sheet S, that is, an inter-image area. In this case, when
the transfer electric field acts on the intermediate transfer belt
B, the developer becomes transferred onto the second-transfer
roller T2b instead of a recording sheet S. Thus, the developer
adheres to the outer surface of the second-transfer roller T2b,
thus contaminating or staining the outer surface of the
second-transfer roller T2b. Therefore, for example, when
transferring a visible image onto a subsequent recording sheet S,
the face of the recording sheet S facing toward the second-transfer
roller T2b, that is, the reverse face of the sheet S, may become
contaminated or stained by coming into contact with the
second-transfer roller having the developer adhered thereon.
Furthermore, when the developer or a paper particle adheres to the
second-transfer roller T2b, the resistance of the second-transfer
roller T2b increases. Thus, a predetermined transfer electric field
is not formed, possibly leading to a transfer defect and
deterioration in image quality.
In the fourth exemplary embodiment, the brush device 101 and the
blade device 111 are disposed so that the surface of the
second-transfer roller T2b is cleaned.
In the brush device 101, the electrostatic brush 103 rotates so as
to clean the second-transfer roller T2b. Specifically, when the
outer surface of the second-transfer roller T2b passes through the
cleaning position Q101, the brush portion 103b removes extraneous
matter, such as a developer, from the second-transfer roller T2b
and collects such extraneous matter, such as a developer, by
adsorption using an electrostatic force generated between the
second-transfer roller T2b and the electrostatic brush 103. When
the electrostatic brush 103 rotationally moves from the cleaning
position Q101, the lubricant 104 is supplied from the supplying
section 104+106. Then, the electrostatic brush 103 supplied with
the lubricant 104 comes into contact with the flicker 107. Thus, a
lubricant excessively supplied to the brush portion 103b, a
developer remaining in the brush portion 103b, and so on are
removed therefrom. Then, when the brush portion 103b returns to the
cleaning position Q101, the brush portion 103b supplies the
lubricant 104 to the second-transfer roller T2b and cleans the
surface of the second-transfer roller T2b.
Furthermore, in the blade device 111, the cleaning blade 113 is in
contact with the surface of the second-transfer roller T2b with a
predetermined contact pressure. Thus, extraneous matter, such as a
developer, is scraped off from the surface of the rotating
second-transfer roller T2b. The outer surface of the
second-transfer roller T2b is supplied with the lubricant 104 at
the cleaning position Q101. Therefore, the lubricant 104 is
supplied from the first cleaning position Q101 to the second
cleaning position Q102, whereby excessive friction is reduced
between the cleaning blade 113 and the second-transfer roller T2b.
Thus, friction of the cleaning blade 113 is reduced.
Consequently, in the fourth exemplary embodiment, contamination of
the reverse face of a recording sheet S caused by extraneous
matter, such as a developer, adhered on the second-transfer roller
T2b may be reduced. Moreover, deterioration in image quality caused
by a change in resistance value of the second-transfer roller T2b
due to the developer may be reduced.
FIGS. 21A and 21B illustrate a comparison between the fourth
exemplary embodiment of the present invention and the related art.
Specifically, FIG. 21A illustrates the operation of the
second-transfer roller according to the fourth exemplary
embodiment, and FIG. 21B illustrates a second-transfer roller
according to the related art.
Referring to FIG. 21B, in a transfer roller 01 according to the
related art in which .tau.s>.tau.v, electric current is less
likely to flow along an outer surface 02 of the transfer roller 01.
Specifically, in the transfer roller 01 according to the related
art, even when a transfer electric field is effective, an electric
potential in accordance with the transfer electric field tends to
occur only within a nip region 03, whereas the electric potential
is less likely to change outside the nip region 03. Thus, assuming
that an electrostatic brush 04 similar to the electrostatic brush
103 according to the fourth exemplary embodiment is electrically
connected to ground, a potential difference between the outer
surface 02 of the transfer roller 01 and the electrostatic brush 04
is small. Therefore, an electric field that causes the developer Tn
to move from the second-transfer roller T2b to the electrostatic
brush 103 is less likely to be generated. Consequently, when using
the transfer roller 01 according to the related art, it is
difficult to collect the developer Tn by simply electrically
connecting the electrostatic brush to ground. Thus, in the
configuration in which the electrostatic brush 04 is disposed
relative to the second-transfer roller 01 according to the related
art, a power source that generates a cleaning electric field, which
causes the developer to be adsorbed to the electrostatic brush, may
be necessary.
In contrast, in the second-transfer roller T2b according to the
fourth exemplary embodiment, the time constant .tau.s in the
surface direction and the time constant .tau.v in the volume
direction satisfy the relationship expressed by expression (11).
Specifically, the relationship .tau.s<.tau.v is satisfied. Thus,
in the second-transfer roller T2b according to the fourth exemplary
embodiment, electric current flows readily along the outer surface
9a of the second-transfer roller T2b. In other words, when electric
current flows between the nip region 16 and the shaft 6, the
electric current flows readily even in a bypassing state.
Specifically, referring to FIG. 21A, when a transfer electric field
is effective, an area where an electric potential in accordance
with the transfer electric field occurs spreads not only in the nip
region 16 but also outside the nip region 16. In other words,
spreading of the electric potential is achieved. Thus, when the
electrostatic brush 103 is electrically connected to ground, a
potential difference occurs between the area where the electric
potential has spread and the electrostatic brush 103, whereby an
electric field E11 is generated.
The electric potential of the electrostatic brush 103 corresponds
to the electric potential of the ground-connected shaft 6 of the
second-transfer roller T2b. Thus, the electric field E11
corresponds to the polarity of electric field extending from the
outer surface 9a of the second-transfer roller T2b toward the shaft
6. Specifically, the electric field E11 corresponds to the polarity
of the transfer electric field. Thus, when the electric field E11
acts on the developer adhered on the second-transfer roller T2b,
the developer tends to move from the outer surface of the
second-transfer roller T2b toward the electrostatic brush 103.
Therefore, the electric field E11 acts as a cleaning electric
field. Consequently, in the fourth exemplary embodiment in which
the second-transfer roller T2b that satisfies expression (11) is
used, the developer may be electrostatically adsorbed readily by
ground connection without having to provide a cleaning power
source. Thus, in the fourth exemplary embodiment, the developer may
be readily removed and cleaned off from the second-transfer roller
T2b with a simple configuration, as compared with the configuration
in the related art in which .tau.s>.tau.v.
In particular, in the fourth exemplary embodiment, an arrangement
distance La of the electrostatic brush 103 satisfies expression
(41). Expression (41) is an experimentally-determined expression
that expresses the arrangement distance La that readily causes the
cleaning electric field E11 to become larger. Therefore, in the
fourth exemplary embodiment, the cleaning electric field E11 tends
to become larger, as compared with a case where expression (41) is
not satisfied, so that the developer may be removed readily from
the second-transfer roller T2b. In other words, cleanability of the
electrostatic brush 103 is improved.
Expression (41) will now be described. When the second-transfer
roller T2b satisfies expression (11), an electric potential in
accordance with the transfer electric field tends to occur also
outside the nip region 16 in the second-transfer roller T2b.
However, the magnitude of the electric potential decreases with
increasing distance from the nip region 16, and an absolute value
of the electric potential at the outer surface 9a of the
second-transfer roller T2b becomes small. Thus, when the cleaning
position Q101 is far away from the nip region 16, the potential
difference between the electrostatic brush 103 and the
second-transfer roller T2b tends to decrease. Therefore, the
electric field E11 also tends to become small. Consequently, it may
sometimes be difficult to improve cleanability of the electrostatic
brush 103 depending on how the electric potential spreads from the
nip region 16. Thus, a particularly desired condition for the
position at which the electrostatic brush 103 is arranged, that is,
the arrangement distance La, is defined.
FIG. 22 illustrates the relationship between a potential difference
between the transfer roller and the electrostatic brush and the
remaining amount of developer.
First, with regard to the potential difference between the
electrostatic brush 103 and the second-transfer roller T2b, a
particularly desired potential difference for cleaning will be
discussed. A desired potential difference is experimentally
determined. Specifically, a visible image of 4.5 g/m.sup.2, that
is, a toner patch equivalent to Cin 100%, is adhered onto the
surface of the transfer roller. Then, the adhered toner patch is
removed by the electrostatic brush 103. In this case, the
relationship between the potential difference between the surface
of the second-transfer roller T2b and the electrostatic brush 103
and the amount of developer remaining on the surface of the
second-transfer roller T2b is checked. FIG. 22 illustrates obtained
results. Referring to FIG. 22, it is confirmed that the remaining
amount of developer decreases drastically as the potential
difference increases from 0 V. However, a change in decrease in the
remaining amount becomes smaller as the potential difference
becomes larger than or equal to 25 V. Then, when the potential
difference is larger than or equal to 50V, the change in decrease
also becomes small in a state where the remaining amount is close
to zero. Thus, when the potential difference is larger than or
equal to 50V, it is confirmed that the remaining amount of
developer is small and that cleanability of the electrostatic brush
103 is high. In this state, the electrostatic brush 103 according
to the fourth exemplary embodiment is connected to ground. Thus, it
is conceivable that the desired condition is that the absolute
value of the electric potential on the second-transfer roller T2b
at the cleaning position Q101 is higher than 50 V.
Furthermore, with regard to the magnitude of the electric potential
occurring in the nip region 16 of the second-transfer roller T2b, a
minimum value thereof is normally 100 V in an actual device. Thus,
the magnitude of the electric potential in the nip region 16 may be
considered to be higher than or equal to 100 V. In many cases, it
is conceivable that the electric potential of the nip region 16 is
higher than 100 V, and that the electric potential in the area
outside the nip region 16 also increases.
When a voltage of 100 V is applied, a distance L50 from the voltage
application position to a position at which the magnitude of the
electric potential decreases to 50 V is measured. Specifically,
with reference to the distance L50 when 100 V is applied, the
arrangement distance La may be set to be shorter than the reference
distance L50 so that particularly favorable cleanability of the
electrostatic brush 103 may conceivably be obtained in normal
use.
However, the spreading of electric potential varies depending on
the time constants .tau.s and .tau.v of the second-transfer roller
T2b. Specifically, flowability of electric current in the volume
direction decreases with increasing .tau.v of the second-transfer
roller T2b. Furthermore, a change in electric potential in the
surface direction becomes smaller with decreasing .tau.s.
Therefore, an electrical change in the surface direction becomes
faster with increasing ratio .tau.v/.tau.s, thus making the
electric potential spread readily in the surface direction.
Consequently, it is conceivable that a desired arrangement position
changes in accordance with the ratio .tau.v/.tau.s of the time
constants of the second-transfer roller T2b.
An experiment for measuring the relationship between the ratio
.tau.v/.tau.s and the distance L50 is performed.
FIG. 23 illustrates a measurement method for measuring a change in
electric potential of the transfer roller.
Referring to FIG. 23, in the experiment for measuring the
relationship between the ratio .tau.v/.tau.s and the distance L50,
a transfer roller in which .tau.s and .tau.v have been adjusted is
used. In the experiment, metallic plates 61' and 62' similar to the
metallic plates 61 and 62 used for measuring the time constants
.tau.s and .tau.v are used for measuring the electric potential.
Specifically, when the time constant .tau.s in the surface
direction and the time constant .tau.v in the volume direction of
the second-transfer roller T2b are displayed as (.tau.s [ms],
.tau.v [ms]), the experiment is performed on three second-transfer
rollers T2b with (3.6, 4.5), (67.6, 80), and (23.9, 26.7),
respectively. The metallic plates 61' and 62' used each have a
thickness of 2 mm. The metallic plates 61' and 62' are spaced apart
from each other by .lamda.5' and are disposed on the outer surface
9a of the transfer roller. In this case, the metallic plates 61'
and 62' are pressed against the outer surface 9a of the roller
layer 7 such that they are engaged therewith by 0.2 mm.
Furthermore, a surface electrometer 66' is disposed facing the
second metallic plate 62'. Then, a voltage of -100 is applied to
the first metallic plate 61'. In this case, the peripheral length
.lamda.5' between the right surface 61b' of the first metallic
plate 61' and the left surface 62b' of the second metallic plate
62', at which the surface potential of the second metallic plate
62' becomes -50 V, is measured as L50.
FIGS. 24A and 24B illustrate the measurement results obtained in
accordance with the fourth exemplary embodiment. Specifically, FIG.
24A illustrates a time constant in the surface direction and a time
constant in the volume direction, and FIG. 24B illustrates the
relationship between the ratio of the time constants and the
reference distance.
The measurement results are shown in FIGS. 24A and 24B. When
.tau.v/.tau.s=1.25, L50 is measured to be 37.7 mm. In this case,
the half-perimeter of .phi.24 is 24.pi./2, and
24.pi./2.apprxeq.37.7. Thus, the distance L50 is equivalent to the
half-perimeter of .phi.24. Therefore, it is confirmed that an
electric potential of 50 V occurs in the entire 180.degree.
rotation-angle range of the second-transfer roller T2b from the nip
region. Consequently, when the second-transfer roller T2b has
.phi.24, if the ratio .tau.v/.tau.s is 1.25 or larger, it is
determined that desired cleanability may be ensured regardless of
whether the electrostatic brush is disposed at any position on the
outer surface of the second-transfer roller T2b.
Furthermore, referring to FIG. 24B, when the ratio .tau.v/.tau.s
becomes smaller than 1.25, it is confirmed that the distance L50
also decreases in accordance with the value of the ratio
.tau.v/.tau.s. In this case, it is confirmed that a linear
relationship is established between the distance L50 and the
time-constant ratio .tau.v/.tau.s. In other words, the distance L50
is obtained as expression (42) shown below:
L50={(.tau.v/.tau.s)/1.25}.times.12.pi. (42)
Thus, in order to determine expression (41), La L50 may be
satisfied. This relationship is a relational expression related to
the perimeter. In this case, if the diameter of the transfer roller
is different, the ratio .tau.v/.tau.s may also change, but the
distance L50 is determined in accordance with the ratio
.tau.v/.tau.s. Therefore, the distance L50 is obtained from the
ratio .tau.v/.tau.s in accordance with the diameter of the transfer
roller. Thus, this is also applicable to a case where the diameter
of the transfer roller is different from .phi.24. Consequently,
expression (41) is obtained as a condition for the arrangement
distance La.
Accordingly, in the fourth exemplary embodiment that satisfies
expression (41), cleanability of the electrostatic brush 103 may be
improved, as compared with a case where expression (41) is not
satisfied.
Furthermore, in the fourth exemplary embodiment, the cleaning blade
113 is also disposed relative to the second-transfer roller T2b.
Specifically, in the fourth exemplary embodiment, the cleaning
blade 113 and the electrostatic brush 103 are both used. Thus, even
if the electrostatic brush 103 is not able to sufficiently clean
the second-transfer roller T2b and the developer remains on the
second-transfer roller T2b, the developer is cleaned off therefrom
by the cleaning blade 113 disposed downstream. Normally, when a
large amount of developer is transported to a cleaning blade, a
portion of the developer moves downstream by sliding under the
cleaning blade. In other words, a cleaning defect occurs.
Therefore, when using a cleaning blade, it is desirable that the
developer moving toward the blade be reduced beforehand. In
contrast, in the fourth exemplary embodiment, the second-transfer
roller T2b cleaned by the electrostatic brush 103 subsequently
moves to the second cleaning position Q102. Thus, the amount of
developer at the second cleaning position Q102 is reduced.
Therefore, the developer removing capability of the cleaning blade
113 is less likely to deteriorate. Consequently, in the fourth
exemplary embodiment, the developer removing capability of the
cleaning blade 113 may be reliably improved, as compared with a
case where a large amount of developer is transported to the
cleaning blade 113. In other words, cleanability may be improved in
the fourth exemplary embodiment.
In addition, in the fourth exemplary embodiment, the surface
roughness Rz of the second-transfer roller T2b is set to be smaller
than or equal to 2.0 .mu.m. In a case where a plate-shaped cleaning
member is used, it is desirable that the contact area between the
edge of the plate, that is, the edge of the blade, and the surface
of the transfer roller be increased. However, when the surface
roughness Rz of the second-transfer roller T2b is larger than 2
.mu.m, it is difficult to increase the contact area. In contrast,
in the fourth exemplary embodiment, the surface roughness Rz is
smaller than or equal to 2 .mu.m, so that the contact area between
the edge of the blade and the surface of the second-transfer roller
T2b may be readily increased. Therefore, contactability between the
cleaning blade 113 and the second-transfer roller T2b may be
readily ensured. Consequently, the developer is less likely to pass
under the blade, whereby cleanability of the cleaning blade may be
improved.
In contrast, in the fourth exemplary embodiment, the electrostatic
brush 103, which is electrically conductive, is connected to ground
so that the cleaning electric field E11 is generated between the
electrostatic brush 103 and the second-transfer roller T2b. In this
electric field E11, transfer electric voltage is utilized so as to
remove the developer from the second-transfer roller T2b. In
addition, the cleaning blade 113 disposed downstream is also used
for removing the developer from the second-transfer roller T2b.
Thus, in the fourth exemplary embodiment, the developer may be
readily removed from the second-transfer roller T2b without having
to switch the polarities of the electric field. Consequently, in
the fourth exemplary embodiment, cleanability with respect to the
second-transfer roller T2b may be readily ensured with a simple
configuration, as compared with a case where a transfer power
source that switches polarities is provided.
Experimental Example 3-1
Next, experiments for checking the effects of the fourth exemplary
embodiment are performed.
In the following description, descriptions regarding configurations
similar to those in the experiments for checking the effects of the
first to third exemplary embodiments will be omitted.
In an experimental example 3-1, an experiment for checking the
effects of the fourth exemplary embodiment is performed by using
the printer U.
With regard to the backup roller T2a, the shaft 1 has a diameter of
14 mm, the roller layer 2 has a thickness of 5 mm, the hardness H1
is an Asker C hardness of 60 degrees, and the volume resistance
value is 6.5 log .OMEGA. at an applied voltage of 1 kV.
With regard to the second-transfer roller T2b, the shaft 6 has a
diameter of 14 mm, and the roller layer 7 has a double-layer
configuration in which the base layer 8 has a thickness of 5 mm and
the surface layer 9 has a thickness of 20 .mu.m. The time constant
.tau.s in the surface direction of the second-transfer roller T2b
according to the experimental example 3-1 is set to 23.9 ms. The
time constant .tau.v in the volume direction is set to 26.7 ms. The
time constants .tau.s and .tau.v are adjusted by independently
controlling the blending of electrical-conductivity additives in
the base layer 8 and the surface layer 9. Furthermore, in the
second-transfer roller T2b according to the experimental example
3-1, the surface roughness Rz is set to 1 .mu.m.
With regard to the electrostatic brush 103, the shaft 103a has a
diameter of 5 mm, 2-denier nylon thread with a length of 2.5 mm is
implanted with a density of 120 kF/inch.sup.2 in the shaft 103a.
The nylon thread has a thread resistance of 7.5 log .OMEGA. at an
applied voltage of 1 kV. Furthermore, the electrostatic brush 103
is also used as a supplying member for applying the lubricant 104.
The lubricant 104 used is composed of ZnSt. The shaft 103a is
electrically connected to ground. The arrangement distance La of
the electrostatic brush 103 according to the experimental example
3-1 is set to 30 mm. Since .tau.s=23.9 and .tau.v=26.7,
{(.tau.v/.tau.s)/1.25}.times.12.pi..apprxeq.33.7. Thus,
La=30<33.7, so that the arrangement distance La satisfies
expression (41).
The second-transfer load is set to 6.4 kgf.
The evaluation experiment is performed at an ambient temperature of
22.degree. C. and a relative humidity of 55%.
In the evaluation method according to the experimental example 3-1,
50,000 sheets of size-A3 J paper are successively fed as evaluation
paper while setting the rotation speed v of the second-transfer
roller T2b at 528 mm/s. In this case, a 20.times.20 [mm.sup.2]
toner patch equivalent to Cin 100% (9.0 g/m.sup.2) for each of YMCK
colors and a 20.times.20 [mm.sup.2] toner patch equivalent to Cin
200%(9.0 g/m.sup.2) for each of RGB colors are formed in the
inter-image area between recording sheets S. The voltage to be
applied by the power source E1 is applied while performing control
such that the transfer current becomes -110 .mu.A (negative
polarity) (constant current control). Then, with regard to the last
one of successively-fed sheets of J paper, contamination on the
reverse face thereof, which faces toward the second-transfer roller
T2b, is evaluated.
Experimental Example 3-2
In an experimental example 3-2, the cleaning blade 113 is disposed
downstream of the electrostatic brush 103. The cleaning blade 113
according to the experimental example 3-2 is composed of urethane
rubber with an Asker C hardness of 78 degrees. The engagement
pressure is set to 1.7 gf/mm. The pressing angle is set to
10.degree.. The pressing angle is an angle formed between the
electrostatic brush 103 and the surface of the second-transfer
roller T2b in a state where the electrostatic brush 103 does not
bend. Other conditions and the evaluation method are the same as
those in the experimental example 3-1.
Experimental Example 3-3
In an experimental example 3-3, the surface roughness Rz of the
second-transfer roller T2b is set to 3 .mu.m. Other conditions and
the evaluation method are the same as those in the experimental
example 3-2.
Experimental Example 3-4
In an experimental example 3-4, the surface roughness Rz of the
second-transfer roller T2b is set to 2 .mu.m. Other conditions and
the evaluation method are the same as those in the experimental
example 3-2.
Experimental Example 3-5
In an experimental example 3-5, the arrangement distance La of the
electrostatic brush 103 is set to 35 mm. Since .tau.s=23.9 and
.tau.v=26.7, {(.tau.v/.tau.s)/1.25}.times.12.pi..apprxeq.33.7.
Thus, La=35>33.7, so that the arrangement distance La in the
experimental example 3-5 does not satisfy expression (41). Other
conditions and the evaluation method are the same as those in the
experimental example 3-1.
Comparative Example 3-1
In a comparative example 3-1, the ground connection of the shaft
103a of the electrostatic brush 103 is released. Specifically, the
shaft 103a is not connected to a power source and is not connected
to ground. In other words, the shaft 103a is in a floating state.
Other conditions and the evaluation method are the same as those in
the experimental example 3-1.
Experimental Results of Experimental Examples 3-1 to 3-5 and
Comparative Example 3-1
FIG. 25 illustrates conditions and experimental results of the
experimental examples 3-1 to 3-5 and the comparative example
3-1.
Referring to FIG. 25, contamination on the reverse face of
evaluation paper is evaluated based on visual observation or
observation using a loupe having 25.times. magnification as an
example of a magnifying glass. If contamination on the reverse face
of evaluation paper is clearly confirmable based on visual
observation, an "x" is given. If contamination on the reverse face
of evaluation paper is confirmable based on visual observation but
is minor, a triangle is given. If contamination on the reverse face
of evaluation paper is not confirmable based on visual observation
but if minor adhesion of toner on the reverse face of evaluation
paper is confirmable based on observation using a loupe, a circle
is given. If contamination on the reverse face of evaluation paper
is not confirmable based on visual observation and adhesion of
toner is not confirmable based on observation using a loupe, a
double circle is given. In other words, an "x" indicates a
non-permissible level, and contamination on the reverse face
decreases in the following order: triangle, circle, double
circle.
Referring to FIG. 25, in the experimental examples 3-1 to 3-5 in
which the second-transfer roller T2b satisfying .tau.s<.tau.v is
used and in which the electrostatic brush 103 is electrically
connected to ground, evaluation results indicating a triangle,
circles, and double circles are obtained. In contrast, in the
comparative example 3-1 in which the second-transfer roller T2b
satisfying .tau.s<.tau.v is used but in which the electrostatic
brush 103 is in a floating state, an evaluation result indicating
an "x" is obtained. Therefore, it is confirmed that, when the
second-transfer roller T2b satisfies expression (11), the
second-transfer roller T2b is cleaned by electrically connecting
the electrostatic brush 103 to ground.
In particular, in each of the experimental examples 3-1 to 3-4 in
which the electrostatic brush 103 is connected to ground and the
arrangement distance La satisfies the relationship expressed by
expression (41), the evaluation result of reverse-face
contamination indicates a circle or a double circle. In contrast,
in the experimental example 3-5 in which the electrostatic brush
103 is connected to ground and the arrangement distance La does not
satisfy the relationship expressed by expression (41), the
evaluation result of reverse-face contamination indicates a
triangle. Therefore, it is confirmed that, even when the
second-transfer roller T2b satisfies expression (11) and the
electrostatic brush 103 is connected to ground, cleanability with
respect to the second-transfer roller T2b may be more improved when
the arrangement distance La satisfies the relationship expressed by
expression (41).
With further reference to the experimental results, the
electrostatic brush 103 is in a floating state in the Comparative
Example 3-1. Thus, the potential difference is less likely to
spread relative to the surface potential of the second-transfer
roller T2b, as compared with a case where the electrostatic brush
103 is connected to ground. Therefore, in the comparative example
3-1, it is determined that the electric field E11 is less likely to
occur. Furthermore, in the experimental example 3-5, the cleaning
position Q101 is far away from the nip region 16 so that the
electric potential at the cleaning position Q101 is low. Thus, in
the experimental example 3-5, it is determined that, even when the
electrostatic brush 103 is connected to ground, the potential
difference between the electrostatic brush 103 and the
second-transfer roller T2b is less likely to spread. In other
words, although the experimental example 3-5 achieves an improved
evaluation of evaluation paper relative to the comparative example
3-1, it is determined that the cleaning electric field E11 is
small. In contrast, in the experimental examples 3-1 to 3-4, it is
determined that a sufficient potential difference occurs between
the second-transfer roller T2b and the electrostatic brush 103 so
that a large cleaning electric field E11 is generated.
As compared with the experimental example 3-1, reverse-face
contamination is suppressed in the experimental examples 3-2 and
3-4 in which the cleaning blade 113 is additionally used.
Therefore, it is confirmed that cleanability may be further
improved in the configuration in which the cleaning blade 113 and
the electrostatic brush 103 are both used. However, although the
cleaning blade 113 is additionally used in the experimental example
3-3, the evaluation result thereof is not improved as much as those
in the experimental examples 3-2 and 3-4. It is determined that
this is due to the surface roughness Rz of the second-transfer
roller T2b being larger than 2.0 .mu.m in the experimental example
3-3. Specifically, it is determined that, because the surface of
the transfer roller is rough, contactability between the cleaning
blade 113 and the second-transfer roller T2b is lost, resulting in
reduced cleanability of the cleaning blade 113. Thus, it is
confirmed that the surface roughness Rz of the second-transfer
roller T2b is desirably 2.0 .mu.m or smaller.
In the experimental examples 3-1 to 3-5, the voltage to be applied
by the power source E1 is applied while performing control,
including performing control on the inter-image area, such that the
transfer current value becomes -110 .mu.A (negative polarity)
(constant current control). Specifically, when facing the
inter-image area and also when facing an image area, the switching
of polarities of voltage to be applied to the second-transfer
roller T2b is not performed. Nonetheless, the evaluation results
for reverse-face contamination indicate a triangle, circles, and
double circles. In particular, in the experimental examples 3-1 to
3-4, the evaluation results for reverse-face contamination indicate
circles and double circles. Consequently, it is confirmed that it
may be unnecessary to switch polarities of applied voltage in this
exemplary embodiment.
Fifth Exemplary Embodiment
Next, a fifth exemplary embodiment of the present invention will be
described. In the description of the fifth exemplary embodiment,
components that correspond to those in the first to fourth
exemplary embodiments are given the same reference characters, and
detailed descriptions thereof will be omitted.
The fifth exemplary embodiment differs from the first exemplary
embodiment in the following points but is similar to the first
exemplary embodiment in other points.
FIGS. 26A and 26B illustrate a relevant part of a transfer device
according to the fifth exemplary embodiment of the present
invention. Specifically, FIG. 26A corresponds to FIG. 3, and FIG.
26B illustrates a detach saw.
Referring to FIGS. 26A and 26B, the second-transfer unit T2 as an
example of a transfer device according to the fifth exemplary
embodiment has a second-transfer roller T2b similar to that in the
first exemplary embodiment. Specifically, with regard to the
second-transfer roller T2b according to the fifth exemplary
embodiment, the time constant .tau.s in the surface direction and
the time constant .tau.v in the volume direction are set such that
.tau.s<.tau.v. The contact roller T2c is connected to a power
source E1'. The power source E1' according to the fifth exemplary
embodiment applies voltage with a polarity for transferring a
visible image on the intermediate transfer belt B onto a recording
sheet S. Specifically, the power source E1' according to the fifth
exemplary embodiment applies voltage with the same polarity as the
charge polarity of toner Tn as an example of a developer to the
backup roller T2a via the contact roller T2c. The shaft 6 of the
second-transfer roller T2b is electrically connected to ground.
Referring to FIG. 26A, a detach saw 201 as an example of an
electricity removal member is disposed to the right of the
second-transfer roller T2b. Specifically, the detach saw 201 is
disposed downstream of the nip region 16 in the transport direction
of the recording sheet S. Referring to FIG. 26B, the detach saw 201
according to the fifth exemplary embodiment has a plate-shaped body
portion 201a extending in the front-rear direction. A serrated
sharp portion 201b is formed on the body portion 201a. The sharp
portion 201b has tip ends that are tapered toward the nip region
16. The detach saw 201 is composed of an electrically-conductive
metallic material. The detach saw 201 according to the fifth
exemplary embodiment is connected to a power source E2. The power
source E2 applies, to the detach saw 201, voltage with the same
polarity as the polarity applied to the backup roller T2a by the
power source E1'.
FIG. 27 illustrates an arrangement position of the detach saw
according to the fifth exemplary embodiment of the present
invention.
Referring to FIG. 27, the detach saw 201 according to the fifth
exemplary embodiment is disposed based on a downstream position
Q202, which is located away from a central position Q201 of the nip
region 16 by a predetermined peripheral length Lb in the rotational
direction of the second-transfer roller T2b. Specifically, a half
line K1 is set as an example of an imaginary line extending from a
rotation axis Q203 of the second-transfer roller T2b and passing
through the downstream position Q202. In this case, an end 201b1 of
the sharp portion 201b as an example of an electricity removal
portion is disposed upstream of the half line K1 in the rotational
direction of the second-transfer roller T2b.
The peripheral length Lb is set based on expression (51) shown
below: Lb={(.tau.v/.tau.s)/1.94}.times.6.pi. (51)
The central position Q201 in the fifth exemplary embodiment is set
based on an imaginary line K2 that connects a rotation axis Q204 of
the backup roller T2a, as an example of an opposing member and a
nipping member, and the rotation axis Q203 of the second-transfer
roller T2b. Specifically, a position where the imaginary line K2
and the nip region 16 intersect is set as the central position Q201
of the nip region 16.
Operation of Fifth Exemplary Embodiment
In the printer U according to the fifth exemplary embodiment having
the above-described configuration, when an image is to be recorded
onto a recording sheet S, the second-transfer unit T2 receives a
second-transfer voltage from the power source E1'. Thus, a transfer
electric field in accordance with the second-transfer voltage is
generated between the intermediate transfer belt B and the
second-transfer roller T2b. Therefore, the transfer electric field
acts on a visible image on the intermediate transfer belt B so that
the visible image becomes transferred from the intermediate
transfer belt B to the recording sheet S. In the second-transfer
roller T2b according to the fifth exemplary embodiment, expression
(11) and expression (12) are satisfied. Therefore, the fifth
exemplary embodiment is similar to the first exemplary embodiment
in that concentration of electric discharge may be alleviated, and
transferability onto thick paper may be ensured.
The recording sheet S is electrostatically charged when passing
through the second-transfer region Q4. When the recording sheet S
is electrostatically charged, the electrostatically-charged
recording sheet S receives an electrostatic force. Thus, after the
recording sheet S passes through the nip region 16, the recording
sheet S may sometimes be bent toward the intermediate transfer belt
B. This may cause the recording sheet S to electrostatically attach
to the intermediate transfer belt B, resulting in a so-called paper
jam. In particular, if the recording sheet S is thin paper, the
rigidity, that is, so-called elasticity, of the recording sheet S
is weak, thus increasing the possibility of a jam. Thus, a jam
tends to occur if the recording sheet S remains in an
electrostatically-charged state.
FIGS. 28A to 28C illustrate a comparison between the fifth
exemplary embodiment of the present invention and the related art.
Specifically, FIG. 28A illustrates the operation of the
second-transfer roller T2b according to the fifth exemplary
embodiment, FIG. 28B illustrates a second-transfer roller according
to the related art, and FIG. 28C illustrates a position where the
recording sheet is detached.
Referring to FIG. 28A, in the fifth exemplary embodiment, the
detach saw 201 is disposed downstream of the second-transfer region
Q4 in the sheet transport direction. The detach saw 201 receives
voltage from the power source E2. Thus, a large potential
difference tends to occur between the electrostatically-charged
recording sheet S and the detach saw 201. Therefore, when the
electrostatically-charged recording sheet S passes, electric
discharge occurs between the detach saw 201 and the reverse face of
the recording sheet S, whereby the electric charge is removed from
the recording sheet S. In other words, the detach saw 201 removes
electricity from the recording sheet S. Therefore, in the fifth
exemplary embodiment, an electrostatic force is less likely to
occur between the intermediate transfer belt B and the recording
sheet S, so that a sheet transport defect, such as a jam, may be
reduced.
Referring to FIG. 28B, in the transfer roller 01 in which
.tau.s>.tau.v, even when a transfer electric field is effective,
an electric potential tends to occur only within the nip region 03.
Thus, the electric potential is less likely to change outside the
nip region 03. An electricity removal member 011 receives voltage
with the same polarity as that of the voltage applied to the backup
roller T2a. Therefore, when the electricity removal member 011 is
disposed relative to the transfer roller 01 according to the
related art at a position facing outside the nip region 03, a
potential difference V01 between the outer surface of the transfer
roller 01 and the electricity removal member 011 tends to
increase.
Normally, with regard to an electricity removal member, electricity
removability thereof increases with increasing potential difference
V02 between the reverse face of the recording sheet S and the
electricity removal member. However, when the voltage applied to
the electricity removal member is increased, the potential
difference V01 between the transfer roller and the electricity
removal member also tends to increase. Thus, electric discharge
tends to occur between the transfer roller and the electricity
removal member. When electric discharge occurs between the transfer
roller and the electricity removal member, the charge amount of the
electricity removal member decreases, thus making it difficult to
remove electricity from the recording sheet S. Moreover, electric
discharge occurring between the transfer roller and the electricity
removal member may damage the transfer roller, thus reducing the
lifespan of the transfer roller.
Therefore, in the related-art configuration in which the potential
difference V01 between the outer surface of the transfer roller 01
and the electricity removal member 011 tends to increase, the
voltage is increased by increasing the distance between the outer
surface of the transfer roller 01 and the electricity removal
member 011, or the distance between the reverse face of the
recording sheet S and the electricity removal member is reduced by
increasing the distance between the outer surface 02 of the
transfer roller 01 and the electricity removal member 011. Then,
electricity is removed from the recording sheet S. However, it is
desirable that the removal of electricity from the recording sheet
S start from a position Q205 where a leading edge S1 of the
recording sheet S separates from the second-transfer roller T2b. In
other words, referring to FIG. 28C, it is desirable that the
electricity removal member be disposed at a position near the
position Q205.
A configuration in which an insulating member is disposed between
the electricity removal member and the transfer roller is also
conceivable.
In contrast, in the second-transfer roller T2b according to the
fifth exemplary embodiment, the time constant .tau.s in the surface
direction and the time constant .tau.v in the volume direction
satisfy the relationship expressed by expression (11). Thus, in the
second-transfer roller T2b according to the fifth exemplary
embodiment, when a transfer electric field is effective, an
electric potential tends to also spread outside the nip region 16.
The detach saw 201 receives voltage with the same polarity as that
of the voltage applied to the backup roller T2a. Therefore, the
polarity of the electric potential of the detach saw 201
corresponds to the electric potential of the nip region of the
second-transfer roller T2b and also corresponds to the polarity of
the electric potential spreading outside the nip region 16.
Consequently, the potential difference between the detach saw 201
and the second-transfer roller T2b tends to become small as
compared with a case where the electric potential does not spread.
In other words, in the fifth exemplary embodiment, electric
discharge is less likely to occur. Therefore, in the fifth
exemplary embodiment, the voltage to be applied to the detach saw
201 may be readily increased, and the detach saw 201 may be readily
disposed close to the position Q205 in the nip region 16.
In particular, in the fifth exemplary embodiment, the end 201b1 of
the detach saw 201 is disposed upstream, in the rotational
direction of the second-transfer roller T2b, of the imaginary line
K1 extending through the downstream position Q202 of the peripheral
length Lb defined by expression (51), as shown in FIG. 27.
Expression (51) is an experimentally-determined expression that
expresses a condition in which electric discharge is particularly
less likely to occur. Therefore, in the fifth exemplary embodiment,
electric discharge may be less likely to occur, as compared with a
case where expression (51) is not satisfied.
Expression (51) will now be described. When the second-transfer
roller T2b satisfies expression (11), an electric potential in
accordance with the transfer electric field tends to occur also
outside the nip region 16 in the second-transfer roller T2b.
However, when the position on the outer surface of the
second-transfer roller T2b is different, the electric potential of
the surface of the second-transfer roller T2b varies. Therefore,
there is a possibility that electric discharge may occur readily
depending on how the electric potential spreads from the nip region
16. Thus, a particularly desired condition for the position at
which the detach saw 201 is disposed is defined.
First, with regard to the electricity removal member and the
transfer roller, conditions for electric discharge and potential
difference will be discussed. When the electricity removal member
and the second-transfer roller T2b are positioned the closest to
each other, a distance ds therebetween of 0.5 mm may generally be
considered as the limit in terms of design. Therefore, the distance
ds between the electricity removal member and the transfer roller
tends to be larger than 0.5 mm. When the distance ds is equal to
0.5 mm, electric discharge tends to occur most readily. Thus, with
a potential difference at which electric discharge is less likely
to occur when ds=0.5 mm, electric discharge is less likely to occur
at that potential difference even when ds 0.5 mm. It is
experimentally confirmed that, when the distance ds is equal to 0.5
mm, electric discharge does not occur so long as the potential
difference is lower than or equal to 3 kV. This condition also
satisfies Paschen's Law. Therefore, it is conceivable that a
desirable condition is a condition in which the electricity removal
member is positioned such that the distance ds between the transfer
roller and the electricity removal member is equal to 0.5 mm and
the potential difference between the transfer roller and the
electricity removal member is lower than or equal to 3 kV.
Furthermore, the maximum value of voltage to be applied to the nip
region 16 of the second-transfer roller T2b is normally 10 kV. When
the applied voltage is at maximum, the recording sheet S tends to
be electrostatically charged most readily, and the magnitude of
voltage to be applied to the electricity removal member is also at
maximum. Therefore, it is conceivable that electric discharge tends
to occur between the transfer roller and the electricity removal
member when the magnitude of voltage to be applied to the nip
region 16 is 10 kV.
Assuming that the magnitude of voltage to be applied to the
electricity removal member is set to 10 kV based on the
configuration of a normal power source, when a voltage of 10 kV is
applied to the second-transfer roller T2b, the potential difference
between the second-transfer roller T2b and the electricity removal
member becomes 3 kV or smaller in a range from the voltage
application position to a position at which the magnitude of the
electric potential decreases to 7 kV. Therefore, when a voltage of
10 kV is applied, the peripheral length Lb from the voltage
application position to the position at which the magnitude of the
electric potential decreases to 7 kV is measured. Then, if the
electricity removal member is disposed upstream of the peripheral
length Lb in the rotational direction of the second-transfer roller
T2b, it is conceivable that electric discharge between the
electricity removal member 201 and the second-transfer roller T2b
is suppressed in normal use.
However, the spreading of electric potential varies depending on
the time constants .tau.s and .tau.v of the second-transfer roller
T2b. Therefore, similar to the distance L50 in the fourth exemplary
embodiment, it is conceivable that a desired peripheral length Lb
changes in accordance with the time-constant ratio
.tau.v/.tau.s.
Thus, an experiment for measuring the relationship between the
ratio .tau.v/.tau.s and the peripheral length Lb is performed.
FIG. 29 illustrates a measurement method for measuring a change in
electric potential of the transfer roller according to the fifth
exemplary embodiment of the present invention.
Referring to FIG. 29, in the experiment for measuring the
relationship between the ratio .tau.v/.tau.s and the peripheral
length Lb, a transfer roller in which .tau.s and .tau.v have been
adjusted is used. The measurement experiment according to the fifth
exemplary embodiment is performed in a manner similar to that in
the measurement method for measuring a change in electric potential
of the transfer roller according to the fourth exemplary
embodiment. However, the measurement experiment according to the
fifth exemplary embodiment is performed on five second-transfer
rollers T2b with time constants (.tau.s [ms], .tau.v [ms]) of (3.6,
7), (61.2, 76.3), (57.6, 80), (49.8, 83.4), and (23.9, 26.7),
respectively. The peripheral length .lamda.5' when the surface
potential of the second metallic plate 62' becomes -7 kV by
applying a voltage of -10 kV to the first metallic plate 61' is
measured such that Lb=.lamda.5'. Since other points are the same as
those in the measurement according to the fourth exemplary
embodiment, a detailed description of the measurement experiment
according to the fifth exemplary embodiment will be omitted.
FIGS. 30A and 30B illustrate the measurement results obtained in
accordance with the fifth exemplary embodiment. Specifically, FIG.
30A illustrates a time constant in the surface direction and a time
constant in the volume direction, and FIG. 30B illustrates the
relationship between the ratio of the time constants and the
peripheral length.
The measurement results are shown in FIGS. 30A and 30B. Referring
to FIG. 30A, when the time constants are (3.6, 7), that is, when
.tau.v/.tau.s=1.94, the peripheral length Lb is measured to be
18.85 mm. In this case, the quarter-perimeter of .phi.24 is
24.pi./4, and 24.pi./4.apprxeq.18.85. Thus, the peripheral length
Lb is equivalent to the quarter-perimeter of .phi.24. Specifically,
it is confirmed that an electric potential of 7 kV or higher occurs
in a 90.degree. rotation-angle range of the second-transfer roller
T2b from the nip region 16. Consequently, when the second-transfer
roller T2b has .phi.24, if the ratio .tau.v/.tau.s is 1.94 or
larger, it is determined that electric discharge between the
second-transfer roller T2b and the electricity removal member is
particularly reduced by disposing the electricity removal member
within the 90.degree. rotation-speed range of the second-transfer
roller T2b from the nip region 16.
Furthermore, referring to FIG. 30B, when the ratio .tau.v/.tau.s
becomes smaller than 1.94, it is confirmed that the peripheral
length Lb also decreases in accordance with the value of the ratio
.tau.v/.tau.s. In this case, it is confirmed that a linear
relationship is established between the peripheral length Lb and
the time-constant ratio .tau.v/.tau.s. In other words,
approximation is possible based on a straight line. Therefore, with
regard to the position at which the electricity removal member is
disposed, the peripheral length Lb for defining a particularly
desired condition is obtained as expression (51) shown below:
Lb={(.tau.v/.tau.s)/1.94}.times.6.pi. (51)
Thus, in the fifth exemplary embodiment in which the end 201b1 of
the detach saw 201 is disposed upstream, in the rotational
direction of the second-transfer roller T2b, of the imaginary line
K1 extending through the downstream position Q202 defined by Lb,
electric discharge is less likely to occur, as compared with a case
where the end 201b1 is disposed downstream of the imaginary line
K1. Therefore, the voltage of the detach saw 201 may be readily
increased, and the detach saw 201 may be readily brought closer to
the position Q205 by being disposed closer toward the nip region
16. Consequently, in the fifth exemplary embodiment, electricity
removability may be readily improved.
In the related art, there is a configuration that performs
transferring onto a recording sheet S by attaching the recording
sheet S to an endless belt member, that is, a so-called transport
belt, in the second-transfer region Q4, and transporting the
recording sheet S thereon. In such a configuration that uses the
transport belt, a jam caused by the sheet S attaching to the
intermediate transfer belt B is less likely to occur. However, this
configuration that uses the transport belt has a larger number of
components than the configuration that uses the transfer
roller.
In contrast, in the fifth exemplary embodiment, the second-transfer
roller T2b is used in the second-transfer region Q4. Moreover, the
detach saw 201 removes electricity from the recording sheet S
passing through the second-transfer region Q4.
Experimental Example 4-1
Next, experiments for checking the effects of the fifth exemplary
embodiment are performed.
In the following description, descriptions regarding configurations
similar to those in the experiments for checking the effects of the
first to third exemplary embodiments will be omitted.
In an experimental example 4-1, an experiment for checking the
effects of the fifth exemplary embodiment is performed by using the
printer U.
With regard to the backup roller T2a, the shaft 1 has a diameter of
14 mm, the roller layer 2 has a thickness of 5 mm, and the volume
resistance value is 8.0 log .OMEGA. at an applied voltage of 1
kV.
With regard to the second-transfer roller T2b, the shaft 6 has a
diameter of 14 mm, and the roller layer 7 has a double-layer
configuration in which the base layer 8 has a thickness of 5 mm and
the surface layer 9 has a thickness of 20 .mu.m. Furthermore, in
the experimental example 4-1, the volume resistance value Rv of the
second-transfer roller T2b is set to 7.5 log .OMEGA. at an applied
voltage of 1 kV. The time constant .tau.s in the surface direction
of the second-transfer roller T2b according to the experimental
example 4-1 is set to 3.6 ms. The time constant .tau.v in the
volume direction is set to 7 ms. The time constants .tau.s and
.tau.v are adjusted by independently controlling the blending of
electrical-conductivity additives in the base layer 8 and the
surface layer 9.
The detach saw 201 is disposed such that the end 201b1 is
positioned on an imaginary line K1' extending through the position
at which the peripheral length from the central position Q201 is
16.9 mm and also through the rotation axis Q203. The distance ds
between the second-transfer roller T2b and the detach saw 201 is
set to 0.5 mm. According to expression (51), in the second-transfer
roller T2b according to the experimental example 4-1,
Lb={(7/3.6)/1.94}.times.6.pi.=18.89. Thus, Lb=18.89>16.9. In the
experimental example 4-1, the detach saw 201 is positioned upstream
of the imaginary line K1 of the second-transfer roller T2b in the
rotational direction of the second-transfer roller T2b.
The second-transfer load is set to 6.4 kgf.
The evaluation experiment is performed at an ambient temperature of
10.degree. C. and a relative humidity of 15%.
In the evaluation method according to the experimental example 4-1,
it is checked whether or not a paper jam and electric discharge
have occurred. In detail, an image forming process is performed
with the second-transfer roller T2b rotating at a rotation speed of
528 mm/s. Specifically, while changing the condition of a voltage
Vd applied to the detach saw 201, duplex printing is performed on
evaluation paper under each condition of the voltage Vd. Duplex
printing is performed on 50 sheets of 52-gsm plain paper and 50
sheets of 64-gsm coated paper as the evaluation paper. Then, it is
checked whether or not a sheet transport defect, that is, a jam,
has occurred in the second-transfer region Q4. Moreover, it is
checked whether or not electric discharge has occurred from the
detach saw 201 to the second-transfer roller T2b. The occurrence of
electric discharge is checked based on whether or not a drastic
change in electric current has occurred by installing an ammeter
between the detach saw 201 and the power source. In addition, the
occurrence of spark discharge is also checked by using a
high-sensitivity camera as an example of an observation device. The
voltage Vd is changed in units of 1 kV between -3 kV and -10
kV.
Experimental Example 4-2
In an experimental example 4-2, the time constant .tau.s in the
surface direction of the second-transfer roller T2b is set to 57.6
ms. The time constant .tau.v in the volume direction is set to 80
ms. According to expression (51), in the second-transfer roller T2b
according to the experimental example 4-2,
Lb={(80/57.6)/1.94}.times.6.pi.=13.49. Thus, Lb=13.49<16.9. In
the experimental example 4-2, the detach saw 201 is disposed
downstream of the imaginary line K1 of the second-transfer roller
T2b in the rotational direction of the second-transfer roller T2b.
The voltage Vd is changed in units of 1 kV between -3 kV and -7 kV.
Other conditions and the evaluation method are the same as those in
the experimental example 4-1.
Experimental Example 4-3
In an experimental example 4-3, the time constant .tau.s in the
surface direction of the second-transfer roller T2b is set to 23.9
ms. The time constant .tau.v in the volume direction is set to 26.7
ms. According to expression (51), in the second-transfer roller T2b
according to the experimental example 4-3,
Lb={(26.7/23.9)/1.94}.times.6.pi.=10.85. Thus, Lb=10.85<16.9. In
the experimental example 4-3, the detach saw 201 is disposed
downstream of the imaginary line K1 of the second-transfer roller
T2b in the rotational direction of the second-transfer roller T2b.
The voltage Vd is changed in units of 1 kV between -3 kV and -6 kV.
Other conditions and the evaluation method are the same as those in
the experimental example 4-1.
Comparative Example 4-1
In a comparative example 4-1, the time constant .tau.s in the
surface direction of the second-transfer roller T2b is set to 67.6
ms. The time constant .tau.v in the volume direction is set to 62
ms. Therefore, the second-transfer roller according to the
comparative example 4-1 does not satisfy expression (11). A value
corresponding to Lb is calculated using expression (51) as follows:
Lb={(62/67.6)/1.94}.times.6.pi.=8.91. Voltages of -3 kV and -4 kV
are used as the voltage Vd. Other conditions and the evaluation
method are the same as those in the experimental example 4-1.
Comparative Example 4-2
In a comparative example 4-2, the time constant .tau.s in the
surface direction of the second-transfer roller T2b is set to 67.6
ms. The time constant .tau.v in the volume direction is set to 26.7
ms. A value corresponding to Lb is calculated using expression (51)
as follows: Lb={(26.7/67.6)/1.94}.times.6.pi.=3.84. Voltages of -3
kV, -4 kV, and -5 kV are used as the voltage Vd. Other conditions
and the evaluation method are the same as those in the experimental
example 4-1.
Experimental Results of Experimental Examples 4-1 to 4-3 and
Comparative Examples 4-1 and 4-2
FIGS. 31A and 31B illustrate conditions and experimental results of
the experimental example 4-1, the experimental example 4-2, the
experimental example 4-3, the Comparative Example 4-1, and the
comparative example 4-2. Specifically, FIG. 31A illustrates the
conditions, and FIG. 31B illustrates the experimental results.
Referring to FIGS. 31A and 31B, a circle is given when passing of
both plain paper and coated paper is confirmed and electric
discharge has not occurred. A circle with a minus symbol is given
when passing of one of plain paper and coated paper is confirmed
and electric discharge has not occurred, while non-passing of the
other one of plain paper and coated paper, that is, a jam, is
confirmed. An "x" is given when it is confirmed that both plain
paper and coated paper are jammed. A triangle is given when it is
confirmed that electric discharge has occurred between the detach
saw 201 and the second-transfer roller T2b.
Referring to FIG. 31B, in each of the experimental examples 4-1 to
4-3 in which the second-transfer roller T2b satisfying
.tau.s<.tau.v is used, an evaluation result indicating a circle
with a minus symbol or a circle is obtained. Thus, it is confirmed
that there is a case where the detach saw 201 may remove
electricity from the evaluation paper. It is also confirmed that
there is a case where electric discharge does not occur. On the
other hand, in each of the comparative examples 4-1 and 4-2 in
which the transfer roller with .tau.s>.tau.v is used, only an
evaluation result indicating an "x" or a triangle is obtained. In
other words, it is confirmed that there is a possibility that the
second-transfer roller T2b may be damaged due to the occurrence of
a jam or electric discharge. Therefore, it is confirmed that, when
the second-transfer roller T2b satisfies expression (11), electric
discharge is less likely to occur between the second-transfer
roller T2b and the detach saw 201. In other words, it is confirmed
that the detach saw 201 may be more readily disposed closer to the
nip region 16 and the electricity removability may be more readily
improved by using the second-transfer roller T2b satisfying
.tau.s<.tau.v.
With further reference to the experimental results, a jam has
occurred on evaluation paper at -3 kV in all of the experimental
examples 4-1 to 4-3 and the comparative examples 4-1 and 4-2. This
is conceivably due to the fact that the voltage of -3 kV is too
low, making it difficult to remove electricity from the evaluation
paper.
Furthermore, in each of the comparative examples 4-1 and 4-2 in
which the transfer roller with .tau.s>.tau.v is used, electric
discharge occurs between the electricity removal member and the
transfer roller before reaching a potential difference at which
electricity is removed from coated paper.
In the experimental example 4-3 in which the transfer roller
satisfying .tau.s<.tau.v is used, a sheet transport defect
occurs on coated paper when the voltage applied to the detach saw
201 is low. However, sheet transportability is ensured for plain
paper by increasing the applied voltage. If the applied voltage is
further increased, electric discharge is confirmed.
In the experimental example 4-2, a sheet transport defect occurs
when the applied voltage is low. However, sheet transportability is
ensured as the voltage Vd is increased. When the voltage Vd is -6
kV, satisfactory sheet transportability is ensured for both plain
paper and coated paper. However, when the applied voltage is
further increased, electric discharge is confirmed.
In contrast, in the experimental example 4-1 in which the detach
saw 201 is positioned upstream of the imaginary line K1 based on
expression (51) in the rotational direction, neither a jam nor
electric discharge is confirmed even when the applied voltage is
increased. Therefore, in the experimental example 4-1, the voltage
may be readily increased without causing electric discharge to
occur, and the voltage range in which sheet transportability of
thin paper may be readily ensured is wide. Moreover, in the
experimental example 4-1, it is confirmed that the possibility of
damaging the transfer roller is also reduced.
There is a case where alternating-current voltage is applied to the
electricity removal member or alternating-current voltage is
superimposed on direct-current voltage for the purpose of, for
example, suppressing scattering of the developer. This exemplary
embodiment of the present invention is effective for such a case.
Even when alternative current is applied (or alternating current is
superimposed), an average voltage value (direct-current component)
thereof causes electric discharge.
Modifications
Although the exemplary embodiments of the present invention have
been described in detail above, the present invention is not to be
limited to the above exemplary embodiments and permits various
modifications within the technical scope of the invention defined
in the claims. Modifications H01 to H09 will be described
below.
In a first modification H01, the image forming apparatus according
to each of the above exemplary embodiments is not limited to the
printer U, but may be, for example, a copying apparatus, a
facsimile apparatus, or a multifunction apparatus having multiple
functions of such apparatuses. Furthermore, each of the above
exemplary embodiments is not limited to an image forming apparatus
of a multicolor developing type and may alternatively be applied to
a so-called monochrome image forming apparatus.
The second exemplary embodiment relates to an example in which the
surface layer 9' is formed by generating an electric field such
that the electrical-conductivity additive 14 is distributed
lopsidedly toward the outer surface 9a. Alternatively, for example,
in a second modification H02, the electrical-conductivity additive
14 may be distributed lopsidedly toward the outer surface 9a by
utilizing the difference in specific gravity between the resin 13
and the electrical-conductivity additive 14. Furthermore, for
example, in a case where the electrical-conductivity additive 14 is
magnetic, the electrical-conductivity additive 14 may be
distributed lopsidedly toward the outer surface 9a by drawing the
electrical-conductivity additive 14 toward the outer surface 9a by
utilizing magnetic force.
In each of the above exemplary embodiments, the roller layer 7 of
the second-transfer roller T2b has a double-layer structure
constituted of the base layer 8 and the surface layer 9 as an
example. Alternatively, for example, in a third modification H03, a
multilayer structure having three or more layers, such as the base
layer 8, the surface layer 9, and a third layer interposed
therebetween, is also permissible. In this case, it is desirable
that the blending quantities of electrical-conductivity additives
12 and 14 are larger for outer layers.
In each of the above exemplary embodiments, the roller layer 7 of
the second-transfer roller T2b has a double-layer structure
constituted of the base layer 8 and the surface layer 9 as an
example. Alternatively, for example, in a fourth modification H04,
a single-layer structure is also permissible. In this case, the
electrical-conductivity additive 14 may be distributed lopsidedly
toward the outer surface of the single layer such that
.tau.s<.tau.v is achieved.
In the fourth exemplary embodiment, the second-transfer roller T2b
is desirably supplied with the lubricant 104. Alternatively, in a
fifth modification H05, the configuration for supplying the
lubricant 104 may be omitted.
In the fourth exemplary embodiment, the lubricant 104 is desirably
supplied to the second-transfer roller T2b via the electrostatic
brush 103. Alternatively, in a sixth modification H06, a supplying
member that applies the lubricant to the second-transfer roller T2b
may be provided in addition to the electrostatic brush 103 such
that the lubricant is supplied from the supplying member.
In the fifth exemplary embodiment, the detach saw 201 is provided
as an example of the electricity removal member. Alternatively, for
example, in a seventh modification H07, an electricity removal
member that uses a wire, that is, a so-called corotron, may be
used.
In the fifth exemplary embodiment, the detach saw 201 is configured
to receive direct-current voltage as an example. Alternatively, for
example, in an eighth modification H08, the detach saw 201 may
receive alternating-current voltage alone or direct-current voltage
with alternating-current voltage superimposed thereon.
As a ninth modification H09 of the fourth and fifth exemplary
embodiments, the electrostatic brush 103 and the detach saw 201 may
both be disposed relative to the second-transfer roller T2b.
The foregoing description of the exemplary embodiments of the
present invention has been provided for the purposes of
illustration and description. It is not intended to be exhaustive
or to limit the invention to the precise forms disclosed.
Obviously, many modifications and variations will be apparent to
practitioners skilled in the art. The embodiments were chosen and
described in order to best explain the principles of the invention
and its practical applications, thereby enabling others skilled in
the art to understand the invention for various embodiments and
with the various modifications as are suited to the particular use
contemplated. It is intended that the scope of the invention be
defined by the following claims and their equivalents.
* * * * *